I hope that by the end of this answer you'll see why Matthew 10:28 almost certainly proves annihilationism(conditional immortality), or at the very least that it proves eternal conscious torment(henceforth ECT) to be wrong. We'll start with γεέννῃ(Gehenna).
What Is Gehenna?
The Greek word γεέννῃ(Gehenna) is a transliteration of the Hebrew phrase Ge-hinnom, which refers to the valley right outside Jerusalem. The valley is explicitly mentioned over 13 times in the OT as either "the Valley of the Son of Hinnom"(Joshua 15:8; 18:16; 2 Kings 23:10; 2 Chronicles 28:3; 33:6; Jeremiah 7:31-32; 19:2; 6; 32:35), or "the Valley of Hinnom"(Nehemiah 11:30). And there are several allusions to the valley, the most prominent being Jeremiah 31:40 and Isaiah 66:24. We'll analyze substantial mentions of Ge-hinnom in the OT.
But first, I want to clear up something. It's a very popular notion that Gehenna refers to a smouldering garbage dump in the Valley of Hinnom. However, this notion has very little evidence to support it. See these articles here and here. So, what was Jesus talking about when He mentioned Gehenna? For that, we have to go to the Old Testament.
In 2 Chronicles 28:3, we see that king Ahaz sacrificed his children in the fire in Ge-hinnom(c.f. 2 Kings 16:3). Manasseh, like his grandfather Ahaz, also burned his children in Ge-hinnom(2 Chronicles 33:6; c.f. 2 Kings 21:6). Jeremiah 32:35 tells us that "they built high places for Baal in the Valley of Ben Hinnom to sacrifice their sons and daughters to Molech," something the Lord never commanded, nor did it enter His mind, that they should do such a detestable thing and so make Judah sin. Manasseh had "shed very much innocent blood, till he had filled Jerusalem from one end to another"(2 Kings 21:16). Because of this, Yahweh decided to bring massive calamity upon Jerusalem and Judah(2 Kings 21:12). When Josiah(Manasseh's grandson) took the throne, he reformed Judah(2 Kings 23:1-25). 2 Kings 23:10 says that Josiah "desecrated Topheth, which is in the Valley of the Son of Hinnom, that no one might burn his son or his daughter as an offering to Molech." We see that Tophet was the place of fire within the Ge-hinnom where child sacrifices took place. Josiah made sure that no one would sacrifice their children in Ge-hinnom by desecrating it. Have images of utter desolation and destruction come to mind? Don't worry, it's going to get much worse. Ge-hinnom shows up in Jeremiah where it is portrayed as a place where the apostate Jews will be utterly eliminated by God.
Jeremiah 7:30-34 For the sons of Judah have done that which is evil in My sight,” declares the Lord. “They have put their detestable things in the house which is called by My name, to defile it. 31 They have built the high places of Topheth, which is in the Valley of Ben-hinnom, to burn their sons and their daughters in the fire, which I did not command, and it did not come into My mind. 32 “Therefore, behold, days are coming,” declares the Lord, “when it will no longer be called Topheth, or the Valley of Ben-hinnom, but the Valley of the Slaughter; for they will bury in Topheth because there is no other place. 33 The dead bodies of this people will be food for the birds of the sky and for the animals of the earth; and no one will frighten them away. 34 Then I will eliminate from the cities of Judah and from the streets of Jerusalem the voice of joy and the voice of gladness, the voice of the groom and the voice of the bride; for the land will become a site of ruins.
If all this doesn't conjure images of complete eradication and desolation, I don't know what will. Ge-hinnom will be called the "Valley of Slaughter", filled to the brim with the corpses of those who have done evil things(i.e. burn their children), rotting in oblivion and being decomposed by scavengers(birds of sky and animals of land); the intense language gives us the imagery of a bloody battle scene of utmost desolation, with no survivors and dead bodies laid out in the open. It doesn't matter what denomination you are a part of or what theology you adhere to, here is something you cannot deny; corpses are NOT conscious entities; not by any stretch of the imagination. Corpses are thoroughly lifeless and oblivious. As such, they cannot be tormented, not for one second, not for eternity. This same language with reference to Ge-hinnom is employed again in Jeremiah 19. Here are the highlights(but read the whole chapter);
Jeremiah 19:2; 6-7 Then go out to the Valley of Ben-hinnom, which is by the entrance of the Potsherd Gate, and proclaim there the words that I tell you,; therefore, behold, days are coming,” declares the Lord, “when this place will no longer be called Topheth or the Valley of Ben-hinnom, but rather the Valley of Slaughter. 7 And I will frustrate the planning of Judah and Jerusalem in this place, and I will make them fall by the sword before their enemies and by the hand of those who seek their life; and I will make their carcasses food for the birds of the sky and the animals of the earth.
Again, images of total destruction, with (unconscious, lifeless) dead bodies(carcasses) laid in the open for scavengers to finish them off.
Next is Jeremiah 31:40, which unequivocally alludes to Ge-hinnom when it refers to the "valley of dead bodies and ashes" near the Kidron(which is right next to the Valley of Hinnom). The good thing about the total eradication of evil is that there is now room for peace and joy to thrive, which is why the area shall be "holy unto Yahweh". Compare the entirety of Jeremiah 31 to Revelation 20-22. The conception is that Ge-hinnom is a place where evil, and all those who have set their hearts on evil, are annihilated; obliterated from existence. And that because of this, a New World, thoroughly free of all wickedness, can finally flourish! Gehenna eliminates the wicked; it doesn't keep them alive for eternity.
So, in conclusion, the concept of Gehenna(which directly correlates to Ge-hinnom in the OT) does not fit with any notions of eternal conscious existence, whether said existence is one of torment or not. It would not make any sense for Jesus to use Gehenna to signify something completely distinct(e.g. conscious existence that lasts for eternity) from the concepts of Ge-hinnom in the Old Testament. What Gehenna DOES fit with is notions of complete desolation and utter destruction, where there are no conscious survivors(nothing but unconscious, lifeless bodies rotting out in the open until there is nothing left). For a more comprehensive study on Gehenna, see this article here. Now, onto Matthew 10:28.
Can We Reconcile Jesus' Use Of ἀπόλλυμι in Matt. 10:28 with ECT?
Let's take a look at a proposed solution to reconciling(making ECT and Matthew 10:28 compatible) Matt. 10:28 and ECT(or eternal conscious separation without the torment, as some believe).
ἀπόλλυμι is about the spiritual destruction of the soul(ψυχή), not the annihilation of it:
NOTE: Those who follow this interpretation operate under the definition of ψυχή(pronounced psuché) denoting the immortal, conscious part of a person that lives on after death(your inner being with thoughts, emotions, attitudes, etc. that continue to exist after death). I do not think this is what ψυχή refers to in Matthew 10:28(you'll see this when I show the interpretation from the perspective of conditional immortality).
Of course, if Jesus in Matt. 10:28 is talking about literal destruction that causes the soul to cease its existence, then the doctrine of ECT is completely off the table, as it is impossible to experience torment if you are completely unconscious(which you would be if your soul, the immortal, immaterial, conscious part of you, was nonexistent). So, in order to reconcile Matt. 10:28 with ECT, an argument is constructed that goes like this;
(1.) ἀπόλλυμι doesn't always mean to annihilate(to cause to cease to exist permanently).
(2.) Many times ἀπόλλυμι refers to rendering something useless, such that it is unable to fulfil its purpose(e.g. Matthew 9:17, Mark 1:24, Luke 5:37, John 6:27).
(3.) In Matthew 10:28, Jesus is saying that God can render the soul(the immortal, immaterial conscious part of you) useless, unable to fulfil its purpose; not that God can cause the soul to permanently cease to exist.
(4.) The purpose of the soul is to experience love, joy, peace, and satisfaction with God forever. To ἀπόλλυμι the soul is to render it unable to fulfil its purpose, which is accomplished by eternally separating it(soul) from the source of love and peace(God), i.e. to be permanently separated from God. That is what it means for the ψυχή(soul) to be ἀπόλλυμι(destroyed); not to literally destroy(which would render something nonexistent), but to spiritually destroy by rendering the soul completely devoid of any purpose, forever separated from God, the only true source of love and peace.
(5.) Since the soul is not annihilated, but still exists, this does not oppose ECT; there is still room for the soul to be consciously tormented for eternity(or consciously separated from God for eternity, without the torment). Jesus' words in Matthew 10:28 neither prove nor disprove ECT, and are perfectly compatible with it.
It's not bad of an argument. And it works... that is, if you interpret "fear the one who is able to destroy both soul" in a complete vacuum with absolutely no context whatsoever. If you don't, several concerns present themselves. What concerns? Well, let's read Matthew 10:28, this time very, very carefully.
Matthew 10:28 Do not be afraid of those who kill the body but cannot kill the soul. Instead, fear the One who can destroy both soul and body in hell.
What does Jesus first tell us? To "not be afraid of those who kill[ἀποκτεῖναι] the body[σῶμα]", ok stop! What is Jesus saying here? He is saying that men(humans) are capable of depriving the body of life(killing it). Is Jesus talking about men spiritually depriving the body(via separation from God in hell), or literally depriving the body of life(i.e. causing it to be dead)? Is the body literally killed, or spiritually killed? It's literally killed, right? Men are literally capable of putting an end to someone's life by killing(ἀποκτεῖναι) the body(σῶμα). Can we agree on that? Great! What does Jesus go on to say? "But cannot kill(ἀποκτεῖναι) the soul(ψυχή)", ok stop again! Jesus uses the exact same word(ἀποκτεῖναι) to describe the killing of the soul(ψυχή) that He had just used 4 words ago(half a second ago) to describe the killing of the body(σῶμα). Do you think Jesus switched from one definition of ἀποκτεῖναι to another in half a second? That when referencing the body, He's talking about literal killing, but when referencing the soul, He's talking about spiritual killing? Or do you think that Jesus, in using the exact same word within the exact same context merely half a second apart, is talking about the same type of killing when referencing both the body and soul?
And what would Jesus' audience have thought when they heard that("kill the ψυχή")? Half a second ago Jesus was talking about the literal killing of the body(σῶμα), and now Jesus is talking about killing the soul(ψυχή). What do you think His audience would have thought when Jesus said "kill the soul"... Would their brains have switched(in half a second) from one type of killing(literal) to another(spiritual)? Better yet, had they even ever heard of something such as spiritual killing(eternal conscious separation from God)?
It's unthinkable that they would have thought that Jesus was talking about some sort of killing other than actual killing. When they heard Jesus say that men are unable to ἀποκτεῖναι the ψυχή, they would have thought that men are unable to put an end to the actual life of the ψυχή; they would not have thought that men are unable to put an end to the usefulness/purpose of the ψυχή by separating it from God for eternity. Of course, that doesn't prove that that was not what Jesus actually meant. But if that was what Jesus meant, He would have greatly misled His entire audience by making them think that He was speaking about literal killing, when He was, in fact, speaking about spiritual killing in the form of eternal separation from God. I think it's MUCH less of a stretch to say that Jesus was talking about literal killing(instead of spiritual killing) when He used ἀποκτεῖναι.
Ok, now that we know that, let's move on. What does Jesus say next? "Instead, fear the One who can destroy both soul and body in hell." What is Jesus doing here? He is making a contrast between the power/ability of God and the power/ability of men. Jesus is saying that what men CANNOT do, God CAN do; that is why He says, instead. He wants us to fear God instead of men because He(God) can do what they(men) cannot. What was it, again, that they could not do? Oh yeah, kill the ψυχή! So what Jesus is saying is that God has the ability to literally(NOT spiritually) end the life of the ψυχή, while men cannot do so; hence we should fear Him rather than men. It's that simple. Nothing in the context permits Jesus' use of ἀπόλλυμι to denote anything other than actual destruction(causing the permanent ceasing of the life of the ψυχή). But wait! There's more... much, much more.
The examples of where ἀπόλλυμι refers to rendering something unable to fulfil its purpose(i.e. Matthew 9:17, Mark 1:24, Luke 5:37, John 6:27); what do they all have in common? In each one of them, ἀπόλλυμι refers to something that happens to inanimate(lifeless) objects(i.e. a wineskin/bottle and food). But Matthew 10:28 is not dealing with inanimate objects; it's dealing with humans, humans that have life(i.e. Jesus is contrasting what men are able to do to YOU with what God is able to do to YOU).
Whenever ἀπόλλυμι deals with humans, it by far refers to the ceasing of life. Granted, it at times does not refer to the ceasing of life, but to people who have been/could have been lost(e.g. Matthew 10:6; 15:24; Luke 15:24, 32; John 18:9). But is that what Jesus is saying in Matthew 10:28? That God will lose your body and soul in Gehenna? Maybe... But the moments where ἀπόλλυμι refers to the ceasing of person's life vastly outweigh the moments where ἀπόλλυμι refers to people who are/could be lost(e.g. Matthew 2:13; 8:25; 10:39, 42; 12:14; 16:25; 21:41; 22:7; 26:52; 27:20; Mark 3:6; 4:38; 8:35; 9:22; 11:18; 9:12; Luke 6:9; 8:24; 9:24-25; 11:51; 13:33; 17:27, 29, 33; 19:47; 20:16; John 12:25; 1 Corinthians 10:9; Jude 5). Incontrovertibly, the primary use of ἀπόλλυμι when referring to people is to denote the END OF LIFE. It never once refers to rendering a person unable to fulfil their purpose(only to rendering lifeless OBJECTS unable to fulfil their purpose).
Are you convinced by the foregoing that Jesus is NOT talking about the "spiritual destruction" of the ψυχή(i.e. rendering the soul without the capacity to fulfil its purpose, completely devoid of life and love)? Perhaps not. I mean, it is a pretty enticing concept(and I am very good at presenting concepts in an attractive way). But that has no bearing whatsoever on the truth. So, I'm going to show you how ἀπόλλυμι has been used in the Bible(the following examples[Luke 17:27, Luke 17:29, 1 Corinthians 15:18] are just a few. ἀπόλλυμι refers to the end of life countless times in the NT. The following are just a few of the most prominent), and then how it's been used in non-Biblical sources.
Luke 17:27 people were eating, they were drinking, they were marrying, and they were being given in marriage, until the day that Noah entered the ark, and the flood came and destroyed[ἀπόλλυμι] them all.
Two questions... Did anyone, anyone at all, survive this? And did anyone, anyone at all, consciously last for eternity(whether tormented or not, it's all the same)? The answer to both questions is absolutely not. Everyone was completely wiped out; absolutely no survivors. The life of every single creature on land had ceased to exist(Genesis 7:21-23). Agreed? Great! Let's move on.
Luke 17:29 but on the day that Lot left Sodom, it rained fire and brimstone from heaven and destroyed[ἀπόλλυμι] them all.
Two questions... Did anyone, anyone at all, survive this? And did anyone, anyone at all, consciously last for eternity(whether tormented or not, it's all the same)? The answer to both questions is absolutely not. The people were eradicated; decimated; blasted off the face of the earth. Absolutely no one survived God's rainstorm of fire and brimstone. Agreed? Great! Let's move on.
1 Corinthians 15:17-18 And if Christ has not been raised, your faith is futile; you are still in your sins. Then those also who have fallen asleep in Christ have perished[ἀπόλλυμι].
Perished? How would Christians have perished if Jesus was never raised from the dead? Well, if Jesus Himself was never resurrected, our hope of resurrection is nil. If Jesus was never resurrected, then those who died in Him would never be resurrected, as in, they would be dead for eternity... They would be in Sheol for eternity; nonexistent and unconscious for eternity. In that way, they would have perished(permanently ceased to exist). You don't say about someone in Heavenly bliss in paradise(or Abraham's bosom, as some believe paradise refers to) that they would have PERISHED by staying in Heavenly bliss for eternity. I'm sorry, but what exactly is "perished" about being comforted in bliss in paradise/Abraham's bosom for eternity??? That's the exact opposite of "perished". Either Paul completely used the wrong word(and I'm sorry, but "perished" is not a word anyone in their right mind would use to describe the state of those in total bliss in a Heavenly paradise for eternity... It's just not), or being in death for eternity would denote permanent nonexistence. I'd say "perish" is a good word for denoting permanent nonexistence; I don't think it's a good word at all for denoting permanent comfort and bliss in paradise/Abraham's bosom. This one verse provides massive substantiation for BOTH annihilationism and Christian mortalism(by the way, you don't have to be a Christian mortalist in order to be an annihilationist. I know annihilationists that believe in a conscious intermediate state of the dead. An example is Mark Corbett: see his comprehensive video here). Killing two birds with one stone(I also think this verse proves that Paul believed the dead to be unconscious, so maybe 3 birds with one stone). :-) The point is, had Jesus never been resurrected from Sheol, we would never be resurrected from Sheol, and thus would stay in Sheol for eternity, permanently nonexistent and unconscious, and hence "perished"(ἀπόλλυμι). Compare this with John 3:15 where the same word is used to denote what happens to those who don't receive eternal life by believing in Jesus(HINT: It's not permanent separation from God by being tortured for eternity[or not tortured. It the same either way]).
Now I'm going to show how ἀπόλλυμι has been used to describe complete nonexistence by non-Biblical writers. This is solely to make the point that ἀπόλλυμι IS in fact a word used to denote the ceasing of existence. I'm not necessarily agreeing with anything these writers say; I'm merely proving that, if one wanted to denote the permanent nonexistence of something(especially the soul[ψυχή]), ἀπόλλυμι would be the right word to use!
They fear that when the soul[ψυχή] leaves the body it no longer exists anywhere, and that on the day when the man dies it is destroyed and perishes[ἀπόλλυμι], and when it leaves the body and departs from it, straightway it flies away and is no longer anywhere, scattering like a breath or smoke. If it exists anywhere by itself as a unit, freed from these evils which you have enumerated just now, (Phaedo 70a)
How is ἀπόλλυμι used in this context by Plato? Unequivocally to denote the permanent nonexistence of the soul(ψυχή)("no longer exists anywhere", "is destroyed", "is no longer anywhere", etc.). The point is, ἀπόλλυμι can be used to denote permanent nonexistence.
And, Cebes, I believe, granted that the soul is more lasting than the body, but said that no one could know that the soul[ψυχή], after wearing out many bodies, did not at last perish[ἀπόλλυμι] itself upon leaving the body; and that this was death—the destruction of the soul, since the body is continually being destroyed.” (Phaedo 91d)
Once again, ἀπόλλυμι is used to denote nonexistence. Plato talks about the death and destruction of the soul[ψυχή], and how it eventually perishes[ἀπόλλυμι], i.e. how it ceases to exist.
But now all men shrink from that face of death which carries with it insensibility, oblivion, and extinction of knowledge, as being dismal, grim, and dark. And they are discomposed when they hear it said of any one, he is perished[ἀπόλλυμι], or he is gone, or he is no more; (Non Posse 26)
Plutarch(the one who wrote Non Posse) lived several centuries after Plato during the time of the NT authors, and here he used ἀπόλλυμι in the exact same way as Plato, that is, to designate total nonexistence. In this context, it's about those who experience death(which, according to Plutarch, means "insensibility, oblivion, and extinction of knowledge", and who is "gone and no more". Clearly, he is speaking about nonexistence).
So, what have we learned? That ἀπόλλυμι is the word used, both by Biblical and non-Biblical writers, to designate the ceasing of existence(Luke 17:27, Luke 17:29, 1 Corinthians 15:18, Phaedo 70a, Phaedo 91d, Non Posse 26). There are not many words better than ἀπόλλυμι when it comes to describing destruction/perishing(the ceasing of existence). ἀπόλλυμι NEVER refers to "spiritual destruction"(the rendering of a person unable to fulfil their purpose of experiencing love and peace with God by being permanently separated from Him). That ἀπόλλυμι refers, not to literal destruction of the ψυχή, but to the spiritual destruction of the ψυχή(permanent separation from the source of love, joy, and peace, i.e. God) in Matthew 10:28 is not something supported by the context of Matthew 10:28, nor by the use of ἀπόλλυμι in the rest of the Bible. There is no evidence to support the notion that Jesus was speaking about spiritual destruction in Matthew 10:28, and all the evidence to support the notion that Jesus was speaking about literal destruction in Matthew 10:28(see the aforementioned).
So, What Does Matthew 10:28 Really Mean?
For an annihilationists view of Matthew 10:28, check out this article here at Afterlife.co. It's the one I hold to, and I'll explain it here. I've already made an incredibly comprehensive case for ἀπόλλυμι in Matthew 10:28 referring to literal destruction(and not spiritual destruction), so I'm not going to try and prove that again here.
Ok, so, first things first; the notion that ψυχή here(Matthew 10:28) refers to the immaterial, immortal conscious part of a person that lives on after death... What's up with that? I made a massively thorough case for the Hebrew counterpart of ψυχή, נֶפֶשׁ(pronounced nephesh), never referring to the immaterial, immortal conscious part of a person in my answer here. ψυχή is the direct counterpart of נֶפֶשׁ. When Biblical authors used ψυχή, they were referring to the same concepts that נֶפֶשׁ in the Hebrew Bible referred to(and it never referred to an immortal conscious soul in the Hebrew Bible). And one of the most common things נֶפֶשׁ referred to in the Hebrew Bible was the LIFE of a person(or creature). And wouldn't you know it, the life of a person is also one of the most common things ψυχή(the Greek counterpart of נֶפֶשׁ) refers to! (The following are translations taken from the ASV Bible)
Matthew 2:20 Arise and take the young child and his mother, and go into the land of Israel: for they are dead that sought the young child’s life[ψυχή].
Matthew 6:25 Therefore I say unto you, be not anxious for your life[ψυχή], what ye shall eat, or what ye shall drink; nor yet for your body, what ye shall put on. Is not the life[ψυχή] more than the food, and the body than the raiment?
Matthew 10:39 He that findeth his life[ψυχή] shall lose it; and he that loseth his life[ψυχή] for my sake shall find it.
Matthew 16:25-26 For whosoever would save his life[ψυχή] shall lose it: and whosoever shall lose his life[ψυχή] for my sake shall find it. For what shall a man be profited, if he shall gain the whole world, and forfeit his life[ψυχή]? or what shall a man give in exchange for his life[ψυχή]?
Matthew 20:28 even as the Son of man came not to be ministered unto, but to minister, and to give his life[ψυχή] a ransom for many.
Mark 3:4 And he saith unto them, Is it lawful on the sabbath day to do good, or to do harm? to save a life[ψυχή], or to kill? But they held their peace.
Mark 8:35-37 For whosoever would save his life[ψυχή] shall lose it; and whosoever shall lose his life[ψυχή] for my sake and the gospel’s shall save it. For what doth it profit a man, to gain the whole world, and forfeit his life[ψυχή]? For what should a man give in exchange for his life[ψυχή]?
Mark 10:45 For the Son of man also came not to be ministered unto, but to minister, and to give his life[ψυχή] a ransom for many.
Luke 6:9 And Jesus said unto them, I ask you, Is it lawful on the sabbath to do good, or to do harm? to save a life[ψυχή], or to destroy it?
Luke 9:24 For whosoever would save his life[ψυχή] shall lose it; but whosoever shall lose his life[ψυχή] for my sake, the same shall save it.
Luke 12:22-23 And he said unto his disciples, Therefore I say unto you, Be not anxious for your life[ψυχή], what ye shall eat; nor yet for your body, what ye shall put on. For the life[ψυχή] is more than the food, and the body than the raiment.
Luke 14:26 If any man cometh unto me, and hateth not his own father, and mother, and wife, and children, and brethren, and sisters, yea, and his own life[ψυχή] also, he cannot be my disciple.
Luke 17:33 Whosoever shall seek to gain his life shall lose it: but whosoever shall lose his life[ψυχή] shall preserve it.
John 10:11 I am the good shepherd: the good shepherd layeth down his life[ψυχή] for the sheep.
John 10:15 even as the Father knoweth me, and I know the Father; and I lay down my life[ψυχή] for the sheep.
John 10:17 Therefore doth the Father love me, because I lay down my life[ψυχή], that I may take it again.
John 12:25 He that loveth his life[ψυχή] loseth it; and he that hateth his life[ψυχή] in this world shall keep it unto life eternal.
John 13:37-38 Peter saith unto him, Lord, why cannot I follow thee even now? I will lay down my life[ψυχή] for thee. Jesus answereth, Wilt thou lay down thy life[ψυχή] for me? Verily, verily, I say unto thee, The cock shall not crow, till thou hast denied me thrice.
John 15:13 Greater love hath no man than this, that a man lay down his life[ψυχή] for his friends.
All this is in the gospels alone. The point is, the life of a person is one of the most common meanings of ψυχή; the immaterial, immortal conscious part of a person that lives on after death is decidedly NOT one of the common meanings of ψυχή in the NT(that is, when used by Biblical authors), if at all. The annihilationist's interpretation contends that ψυχή in Matthew 10:28 refers to the same thing it does countless other times in the NT, and that it does NOT refer to the immaterial, immortal conscious part of a person that lives on after death.
So essentially, Jesus is saying in Matthew 10:28 that men are merely capable of killing you temporarily by killing your body, and are not capable of permanently taking your life. The One who IS capable of permanently taking our life is God; He alone is sovereign over ALL life. Men can cause our life to cease to exist temporarily(and God can easily bring us back from that), but they have absolutely NO power over the ultimate fate of our lives. God, on the other hand, can cause our life to cease to exist permanently in Gehenna, that is to say, He, and He alone, has the ability to annihilate our lives. Men cannot ultimately kill our lives; only God can, and He does so in Gehenna, the place of utter desolation in which no one can survive(He alone has the authority to cast into Gehenna[Luke 12:5]). This is why we must fear God over men. It is completely illogical to fear those who have limited power over our lives over the One who has ultimate control over our lives! And Jesus' goal is to make sure we know that. So, Matthew 10:28 reads like this;
Matthew 10:28 Do not be afraid of those who can kill your body[temporarily killing you] but cannot [permanently] kill your life[ψυχή]. Instead, fear the One who can [do what men cannot] permanently kill[annihilate] both life[ψυχή] and body in Gehenna[the place of utter desolation, in which no one can survive].
Hope this helps! Have a great day. :)