I don’t speak for all Protestants or any Protestants, I am providing an answer to why gentiles and Israel and Jews are able to enter the New Covenant.
Stephen says in Acts that the ecclesia was at mount Sinai. It’s the same word in the NT to describe the Church. Was the Church at Mount Sinai?
“This is the one who was in the ecclesia (Greek text) in the wilderness together with the angel who was speaking to him on Mount Sinai, and who was with our fathers; and he received living oracles to pass on to you.”
Acts 7:38 NASB
At Mount Sinai we know G-d made a covenant with Israel all twelve tribes and also the strangers/gentiles that were present there too
“And the people of Israel journeyed from Rameses to Succoth, about six hundred thousand men on foot, besides women and children. A mixed multitude also went up with them, and very much livestock, both flocks and herds.”
Exodus 12:37-38 ESV
but Ezekiel says that it was at that time that G-d married Israel/Ekklesia
“"Then I passed by you and saw you, and behold, you were at the time for love; so I spread My skirt over you and covered your nakedness. I also swore to you and entered into a covenant with you so that you became Mine,"declares the Lord G-d.”
Ezekiel 16:8 NASB
We know that under Saul, David and Solomon there was one kingdom but in the reign of Solomon’s son Rehoboam the kingdom is split into the northern ten tribes and the southern two Judah and Benjamin that went by the name Judah.
Northern kingdom was sent into captivity but never returned but beyond that G-d gave them a certificate of divorce. So now the northern kingdom also known as Israel or Efraim, after captivity spread throughout all the known world and became indistinguishable from the gentiles
“And I saw that for all the adulteries of faithless Israel, I had sent her away and given her a writ of divorce, yet her treacherous sister Judah did not fear; but she went and was a harlot also.”
Jeremiah 3:8 NASB
Judah likewise went into captivity but for only 70 years and returned back to the land of G-d. Due to His Covenant to David Judah does not get a certificate of divorce.
Then comes The Christ and brings salvation but now there was a problem, the law of Moses says that a divorced wife cannot return back to her first husband even if the second husband dies because she is bound by the law of her first husband
“then her former husband who sent her away is not allowed to take her again to be his wife, since she has been defiled; for that is an abomination before the LORD, and you shall not bring sin on the land which the LORD your God gives you as an inheritance.”
Deuteronomy 24:4 NASB
But yet the Bible says in multiple passages that He will take back Israel
“How can I give you up, O Ephraim? How can I surrender you, O Israel? How can I make you like Admah? How can I treat you like Zeboiim? My heart is turned over within Me, All My compassions are kindled.”
Hosea 11:8 NASB
And the heavenly hosts must have said to each other, doesn’t G-d remember that He said you cannot remarry once divorced to the same wife? But instead of saying anything they waited to see what would happen
“It was revealed to them that they were not serving themselves, but you, in these things which now have been announced to you through those who preached the gospel to you by the Holy Spirit sent from heaven-things into which angels long to look.”
1 Peter 1:12 NASB
So now we have the dilemma, among the gentiles was Israel and they were the divorced wife of G-d, so how can the gentiles be saved and G-d not sin by remarrying and reentering into Covenant with those who were divorced. Apostle Paul explains
“So then, if while her husband is living she is joined to another man, she shall be called an adulteress; but if her husband dies, she is free from the law, so that she is not an adulteress though she is joined to another man.”
Romans 7:3 NASB
If the first husband dies, the woman is released from the law of her first husband because CHRIST died, releasing Israel but Christ also resurrected saying, pick me, I’m an elegible bachelor. And the ten tribes and the gentiles enter into the New Covenant but the Jews rejected their own Christ/Messiah
Notice a text comparison between the OT and the NT
“Yet the number of the children of Israel shall be like the sand of the sea, which cannot be measured or numbered. And in the place where it was said to them, "You are not my people," it shall be said to them, "Children of the living G-d." And the children of Judah and the children of Israel shall be gathered together, and they shall appoint for themselves one head. And they shall go up from the land, for great shall be the day of Jezreel.”
Hosea 1:10-11 ESV
There is a distinction made between Judah and Israel and they are gathered together because in order to gather they must be apart or dispersed.
See how Apostle Paul says Jews and then ties Gentiles to the passage in Hosea that speaks about Israel or the ten lost tribes
“even us whom he has called, not from the Jews only but also from the Gentiles? As indeed he says in Hosea, "Those who were not my people I will call 'my people,' and her who was not beloved I will call 'beloved.'" "And in the very place where it was said to them, 'You are not my people,' there they will be called 'sons of the living G-d.'"”
Romans 9:24-26 ESV
So I said all that to say this
Jews are the kingdom of Judah, and Judah was older than Efraim the name of the ten tribes. The northern kingdom accepted Christ and came home and could join into Covenant and not just Efraim but the gentiles too just like at Mount Sinai because it’s not a bloodline issue anymore and it didn’t help that Israel/Efraim mixed with the nations of it were a blood line requirement
“who were born, not of blood nor of the will of the flesh nor of the will of man, but of G-d.”
John 1:13 NASB
But Judah refused to come in and complained
“But he answered and said to his father, 'Look! For so many years I have been serving you and I have never neglected a command of yours; and yet you have never given me a young goat, so that I might celebrate with my friends; but when this son of yours came, who has devoured your wealth with prostitutes, you killed the fattened calf for him.'”
Luke 15:29-30 NASB
Efraim mixed with the gentiles, they were not a people anymore but Christ died and released them from the law of the first husband so now they could re-enter into Covenant with Him by faith and because it’s by faith and not bloodline anyone can be saved.
"Behold, the days are coming, declares the Lord, when I will establish a new covenant with the house of Israel and with the house of Judah,”
Hebrews 8:8 ESV
“And in this way all Israel will be saved"
Romans 11:26a ESV
“And as for all who walk by this rule, peace and mercy be upon them, and upon the Israel of G-d.”
Galatians 6:16 ESV
“James, a servant of God and of the Lord Jesus Christ, To the twelve tribes in the Dispersion: Greetings.”
James 1:1 ESV
The Church is practically Israel made up of gentiles who intermingled with the ten lost tribes and the Jews are the older brother who is outside refusing to come inside but the Jews will come too and then all twelve tribes will be saved. It’s Israel and that mixed multitude coming into covenant with the Bridegroom at Mt. Sinai but now under a new Covenant.