The Immortality of the Soul is "the doctrine that the human soul will survive death, continuing in the possession of an endless conscious existence." (source).
What is the biblical basis for the immortality of the soul?
First, we must prove that there is a soul within man which can exist outside the body, i.e. disembodied, for there are some who deny that man has a soul, but instead, they assert that he is a soul (which objection is addressed here).
In 2 Cor. 12:2-4, the apostle Paul wrote,
2 I know a man in Christ fourteen years ago (I don't know whether in the body or outside the body; God knows), such a man was caught away unto the third heaven. 3 And I know such a man (I don't know whether in the body or outside the body; God knows) that he was caught away into Paradise, and he heard unspeakable words which are not lawful for a man to utter.
2 οἶδα ἄνθρωπον ἐν Χριστῷ πρὸ ἐτῶν δεκατεσσάρων εἴτε ἐν σώματι οὐκ οἶδα εἴτε ἐκτὸς τοῦ σώματος οὐκ οἶδα ὁ θεὸς οἶδεν ἁρπαγέντα τὸν τοιοῦτον ἕως τρίτου οὐρανοῦ 3 καὶ οἶδα τὸν τοιοῦτον ἄνθρωπον εἴτε ἐν σώματι εἴτε ἐκτὸς τοῦ σώματος οὐκ οἶδα ὁ θεὸς οἶδεν 4 ὅτι ἡρπάγη εἰς τὸν παράδεισον καὶ ἤκουσεν ἄῤῥητα ῥήματα ἃ οὐκ ἐξὸν ἀνθρώπῳ λαλῆσαι
Henry Alford wrote,
The adoption of the third person is remarkable: it being evident from 2 Corinthians 12:7 that he himself is meant. It is plain that a contrast is intended between the rapt and glorified person of 2 Corinthians 12:2; 2 Corinthians 12:4,—and himself, the weak and afflicted and almost despairing subject of the σκόλοψ τῇ σαρκί of 2 Corinthians 12:7 ff. Such glory belonged not to him, but the weakness did. Nay, so far was the glory from being his, that he knew not whether he was in or out of the body when it was put upon him: so that the ἐγὼ αὐτός, compounded of the νοῦς and σάρξ (Romans 7:25), clearly was not the subject of it, but as it were another form of his personality, analogous to that which we shall assume when unclothed of the body.
It may be remarked in passing, as has been done by Whitby, that the Apostle here by implication acknowledges the possibility of consciousness and receptivity in a disembodied state.
It's evident from that passage that man can have existence outside the body (ἐκτὸς τοῦ σώματος), and if not as a sentient soul, then what? Indeed, it was as a soul, or if you wish, a spirit (for the two are sometimes treated as synonymous), that Lazarus, the rich man, and Abraham (who had died long before) were all in Hades, sensible and sentient (Luke 16:19-31).
Having thus proved that the personality can exist as a soul outside the body (disembodied), sensible and sentient, we must now prove that the soul is immortal.
Justin Martyr summarizes this in his Dialogue with Trypho the Jew (Ch. VI):
For the truth is so; and you would perceive it from this. The soul assuredly is or has life. If, then, it is life, it would cause something else, and not itself, to live, even as motion would move something else than itself. Now, that the soul lives, no one would deny. But if it lives, it lives not as being life, but as the partaker of life; but that which partakes of anything, is different from that of which it does partake. Now the soul partakes of life, since God wills it to live. Thus, then, it will not even partake [of life] when God does not will it to live. For to live is not its attribute, as it is God's; but as a man does not live always, and the soul is not for ever conjoined with the body, since, whenever this harmony must be broken up, the soul leaves the body, and the man exists no longer; even so, whenever the soul must cease to exist, the spirit of life is removed from it, and there is no more soul, but it goes back to the place from whence it was taken.' (Roberts-Donaldson)
τὸ γὰρ ἀληθὲς οὕτως ἔχει· μάθοις δ' ἂν ἐντεῦθεν. ἡ ψυχὴ ἤτοι ζωή ἐστιν ἢ ζωὴν ἔχει. εἰ μὲν οὖν ζωή ἐστιν, ἄλλο τι ἂν ποιήσειε ζῆν, οὐχ ἑαυτήν, ὡς καὶ κίνησις ἄλλο τι κινήσειε μᾶλλον ἢ ἑαυτήν. ὅτι δὲ ζῇ ψυχή, οὐδεὶς ἀντείποι. εἰ δὲ ζῇ, οὐ ζωὴ οὖσα ζῇ, ἀλλὰ μεταλαμβάνουσα τῆς ζωῆς· ἕτερον δέ τι τὸ μετέχον τινὸς ἐκείνου οὗ μετέχει. ζωῆς δὲ ψυχὴ μετέχει, ἐπεὶ ζῆν αὐτὴν ὁ θεὸς βούλεται. οὕτως ἄρα καὶ οὐ μεθέξει ποτέ, ὅταν αὐτὴν μὴ θέλοι ζῆν. οὐ γὰρ ἴδιον αὐτῆς ἐστι τὸ ζῆν ὡς τοῦ θεοῦ· ἀλλὰ ὥσπερ ἄνθρωπος οὐ διὰ παντός ἐστιν οὐδὲ σύνεστιν ἀεὶ τῇ ψυχῇ τὸ σῶμα, ἀλλ', ὅταν δέῃ λυθῆναι τὴν ἁρμονίαν ταύτην, καταλείπει ἡ ψυχὴ τὸ σῶμα καὶ ὁ ἄνθρωπος οὐκ ἔστιν, οὕτως καί, ὅταν δέῃ τὴν ψυχὴν μηκέτι εἶναι, ἀπέστη ἀπ' αὐτῆς τὸ ζωτικὸν πνεῦμα καὶ οὐκ ἔστιν ἡ ψυχὴ ἔτι, ἀλλὰ καὶ αὐτὴ ὅθεν ἐλήφθη ἐκεῖσε χωρεῖ πάλιν. (Patrologiae Cursus Completus. Series Graeca. Vol. 6)
The apostle Paul wrote that only God has immortality (1 Tim. 6:16); therefore, since all else is created by God, all else is not immortal. But, what about those of us who will one day become immortal in incorruptible, spiritual bodies, as the apostle Paul wrote in 1 Cor. 15:53,
For this corruptible must put on incorruption, and this mortal must put on immortality.
δεῖ γὰρ τὸ φθαρτὸν τοῦτο ἐνδύσασθαι ἀφθαρσίαν καὶ τὸ θνητὸν τοῦτο ἐνδύσασθαι ἀθανασίαν
Is not man to be immortal also? Evidently so; thus, we must distinguish between (1) inherent immortality and (2) granted immortality. God alone has inherent immortality, that is, "life in Himself" (John 5:26). But, He grants or gives immortality to those He wills.
For example, in John 10:28, it is written,
And I give them eternal life, and they shall certainly never perish, neither shall anyone pluck them out of my hand.
κἀγὼ ζωὴν αἰώνιον δίδωμι αὐτοῖς καὶ οὐ μὴ ἀπόλωνται εἰς τὸν αἰῶνα καὶ οὐχ ἁρπάσει τις αὐτὰ ἐκ τῆς χειρός μου
It should be noted that immortality is also granted to the unrighteous, but for the purpose of eternal torment rather than eternal life (i.e., reward) (cp. Dan. 12:2)).
9 The Lord God made all kinds of trees grow out of the ground—trees that were pleasing to the eye and good for food. In the middle of the garden were the tree of life and the tree of the knowledge of good and evil.
Before being banished from Eden, eternal life was possible by eating from the Tree of Life. Revelation 22 three times mentions the Tree of Life as present in the new Earth.
25 I know that my redeemer lives, and that in the end he will stand on the earth. 26 And after my skin has been destroyed, yet in my flesh I will see God; 27 I myself will see him with my own eyes—I, and not another. How my heart yearns within me!
2 Multitudes who sleep in the dust of the earth will awake: some to everlasting life, others to shame and everlasting contempt.
But your dead will live, Lord; their bodies will rise— let those who dwell in the dust wake up and shout for joy— your dew is like the dew of the morning; the earth will give birth to her dead.
But God will redeem me from the realm of the dead; he will surely take me to himself.
9 Therefore my heart is glad and my tongue rejoices; my body also will rest secure, 10 because you will not abandon me to the realm of the dead, nor will you let your faithful one see decay. 11 You make known to me the path of life; you will fill me with joy in your presence, with eternal pleasures at your right hand.
20 Though you have made me see troubles, many and bitter, you will restore my life again; from the depths of the earth you will again bring me up.
That is just the Old Testament...
47 For this is what the Lord has commanded us: “‘I have made you a light for the Gentiles, that you may bring salvation to the ends of the earth.’[c]” 48 When the Gentiles heard this, they were glad and honored the word of the Lord; and all who were appointed for eternal life believed.
6 God “will repay each person according to what they have done.” 7 To those who by persistence in doing good seek glory, honor and immortality, he will give eternal life. 8 But for those who are self-seeking and who reject the truth and follow evil, there will be wrath and anger.
Whoever sows to please their flesh, from the flesh will reap destruction; whoever sows to please the Spirit, from the Spirit will reap eternal life.
And many more...
What is the biblical basis for the immortality of the soul?
Romans 6:23 For the wages of sin is death; but the gift of God is eternal life through Jesus Christ our Lord.
John 5:24 Verily, verily, I say unto you, He that heareth my word, and believeth on him that sent me, hath everlasting life, and shall not come into condemnation; but is passed from death unto life.
1 Thessalonians 4:17 Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord.
Revelation 21:3-4 And I heard a great voice out of heaven saying, Behold, the tabernacle of God is with men, and he will dwell with them, and they shall be his people, and God himself shall be with them, and be their God. And God shall wipe away all tears from their eyes; and there shall be no more death, neither sorrow, nor crying, neither shall there be any more pain: for the former things are passed away
1 body + 1 spirit = 1 living soul (Genesis 2:7).
Body without the spirit is dead being alone (James 2:26).
God "has chosen us in Him before the foundation of the world... Having predestinated us unto the adoption of children by Jesus Christ to himself" (Ephesians 1:4-5).
Whosoever believes in the Lord Jesus Christ has eternal life (John 3:15).
Immortal is a term that is often used interchangeably with eternal but not used precisely in its meaning.