At any given time during the history of Israel there were a few of them among the many of their people who were loyal and faithful to God, with whom they had a covenant relationship which granted them privileges and promises no other people on the earth had. It also made them more responsible to be faithful and obedient to him. He had birthed them as a people for a 4-fold purpose that we can discuss later. During their long history there were times that very few faithful among them could be found in Israel. but through these few the hope for Israel was kept alive.These became known as "the remnant." It is a theme of the OT prophets and we find it during the age of grace because Israel has not changed in character and only a few were willing to repent and trust Jesus, the Messiah of Israel, who came to save the whole nation. After his resurrection he called on "every one of them to be saved and, of course, they refused.
Ac 2:38 Then Peter said unto them, Repent, and be baptized "every one of you" in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost.
There is a penalty down the road for refusing.
Acts 3:20 And he shall send Jesus Christ, which before was preached unto you:
21 Whom the heaven must receive until the times of restitution of all things, which God hath spoken by the mouth of all his holy prophets since the world began.
22 For Moses truly said unto the fathers, A prophet shall the Lord your God raise up unto you of your brethren, like unto me; him shall ye hear in all things whatsoever he shall say unto you.
23 And it shall come to pass, that every soul, (in context that is every Israelite soul) which will not hear that prophet, shall be destroyed from among the people.
24 Yea, and all the prophets from Samuel and those that follow after, as many as have spoken, have likewise foretold of these days.
25 Ye are the children of the prophets, and of the covenant which God made with our fathers, saying unto Abraham, And in thy seed shall all the kindreds of the earth be blessed.
26 Unto you first God, having raised up his Son Jesus, sent him to bless you, in turning away every one of you from his iniquities.
Ac 3:19 Repent ye therefore, and be converted, that your sins may be blotted out, when the times of refreshing shall come from the presence of the Lord;
Because of the covenant promises to Israel from God and because he considered the collective nation his "son" his covenant promises will not be fulfilled until everyone of them are saved from their sins and separated from them.
There has not been a conversion of every one of Israel yet, but in order for the restitution of all things there will have to be. This is what the Great Tribulation will accomplish. It will purge Israel of every unbeliever and all that are left alive, though few in number, will be saved and for the first time, and forever, all Israel will be saved.
Here is the dynamic for gentiles, who have never had a covenant promise of salvation directly from God. He is never going to save us nationally, but he does offer salvation through the gospel to each of us. Understand this in the context of the beginning of gentile evangelism and the extent of it; Read this carefully from the KJV.
Acts 15:5 But there rose up certain of the sect of the Pharisees which believed, saying, That it was needful to circumcise them (that is Paul's gentile converts on his first missionary undertaking), and to command them to keep the law of Moses.
6 And the apostles and elders came together for to consider of this matter.
7 And when there had been much disputing, Peter rose up, and said unto them, Men and brethren, ye know how that a good while ago God made choice among us, that the Gentiles by my mouth should hear the word of the gospel, and believe. (he is referring to Acts 10)
8 And God, which knoweth the hearts, bare them witness, giving them the Holy Ghost, even as he did unto us;
9 And put no difference between us and them, purifying their hearts by faith.
10 Now therefore why tempt ye God, to put a yoke upon the neck of the disciples, which neither our fathers nor we were able to bear?
11 But we believe that through the grace of the Lord Jesus Christ we shall be saved, even as they.
12 Then all the multitude kept silence, and gave audience to Barnabas and Paul, declaring what miracles and wonders God had wrought among the Gentiles by them. (the Jews needed to know about these signs to confirm their authenticity-the Jews require a sign)
The reader need to believe these words and follow the logic.
13 And after they had held their peace, James answered, saying, Men and brethren, hearken unto me:
14 Simeon hath declared how God at the first did visit the Gentiles, to take out of them a people for his name.
15 And to this agree the words of the prophets; as it is written,
16 After this (after evangelizing the gentiles, an act that still goes on today) I will return, and (and do what?) will build again the tabernacle of David, which is fallen down; and I will build again the ruins thereof, and I will set it up:
17 That the residue of men might seek after the Lord, and all the Gentiles, upon whom my name is called, saith the Lord, who doeth all these things.
18 Known unto God are all his works from the beginning of the world.
19 Wherefore my sentence is, that we trouble not them, which from among the Gentiles are turned to God:
20 But that we write unto them, that they abstain from pollutions of idols, and from fornication, and from things strangled, and from blood.
The gentiles are not to violate Jewish sensibilities. There are Jew and gentile Christians in the world.
I have more to say about this in the context of James and the 12 tribes of Israel. The doctrines cannot be known without knowing these things.
Mt 19:27 Then answered Peter and said unto him, Behold, we have forsaken all, and followed thee; what shall we have therefore?
28 And Jesus said unto them, Verily I say unto you, That ye which have followed me, in the regeneration when the Son of man shall sit in the throne of his glory, ye also shall sit upon twelve thrones, judging the twelve tribes of Israel.
The 12 tribes of Israel is not a myth. Today, since the resurrection of Jesus Christ, Jewish converts to the Christian faith, are the "remnant" and they have a name in this age one will find in Romans 11:5. Read about it there. (surprise: it would appear in v 5 because that is the number of God for grace)