In the first 33 years of teaching the Gospel of Christ Jesus about His death and resurrection now being able to bring people to salvation, that salvation in Christ was ONLY given to people who were of the House of Israel.
Jews within the Roman and Parthian Empires who lived at the time were certainly of the tribe of Judah who were a part of the House of Israel. It meant that salvation could be given to them, but it could not be given to Gentiles IF THEY REMAINED GENTILES! In the 33 years of time that elapsed from Christ's crucifixion in 30 to 63 C.E., it was necessary for all Gentiles who were saved in Christ to become spiritual Israelites. The New Covenant relationship with God was given only to those who were members of the House of Israel and the House of Judah (Jeremiah 31:31–34). Note what the clear word of God states:
"Behold, the days come, says the Lord, that I will make a new covenant with the house of Israel, and with the house of Judah: Not according to the covenant that I made with their fathers in the day that I took them by the hand to bring them out of the land of Egypt; which my covenant they broke, although I was an husband unto them, says the Lord: But this shall be the covenant that I will make with the house of Israel; After those days, says the Lord, I will put my law in their inward parts, and write it in their hearts; and will be their God, and they shall be my people. And they shall teach no more every man his neighbor, and every man his brother, saying, 'Know the Lord': for they shall all know me, from the least of them unto the greatest of them, says the Lord: for I will forgive their iniquity, and I will remember their sin no more." Jeremiah 31:31–34
As clear as God could make it, the New Covenant was to be made only with the House of Israel and the House of Judah. There is not one mention in this New Covenant relationship about participation of Gentiles who were not Israelites. And what did this lack of mention concerning Gentiles mean to the apostle Paul? He made it clear that salvation in Christ could only be given to the children of Israel and no other nation. Note what Paul said in Ephesians chapter 2:
"Wherefore remember [you Gentiles], that you, being in time past Gentiles in the flesh, who are called Uncircumcision by that which is called the Circumcision in the flesh made by hands; That at that time you were without Christ, being aliens from the commonwealth of Israel, and strangers from the covenants of promise [both Old and New Covenants], having no hope, and without God in the world." Ephesians 2:11–12
Paul made it clear in the above statement, which was absolutely true and accurate, that as long as Gentiles remained Gentiles, there was not a hope in the world for them to gain a salvation in Christ. They were those "having no hope." Remember, the Old and New Covenants were made only with the House of Israel and if one was not an Israelite then such persons were "strangers from the covenants of promise. ... and without God in the world." This was a terrible plight for Gentiles to be in, but this was their fate. Simply put, Gentiles could not be saved.
However, there came to be hope, and indeed, even a victory for the Gentiles. A legal was was found that could get Gentiles saved along with the Israelites. This was by grafting Gentiles into the stock of Israel so that they no longer would be reckoned as Gentiles. This was the manner in which Gentiles could be called the children of Abraham and the Israel of God, and be accepted for salvation like the other Israelites. And Paul, in the four epistles to the first three ekklesias (the congregations in Romans, Corinthians and Galatians) taught that all Gentiles who accepted Christ as their personal Savior were now reckoned to be "in Christ" (Romans 12:5, 16:7, 9–10; 1 Corinthians 3:1, 15:22; 2 Corinthians 1:21, 3:14, 5:17; Galatians 3:27). Putting Gentiles "in Christ" gave them an advantage they had not realized before. Since Christ was clearly reckoned in a legal sense to be from Abraham, Isaac and Jacob (Jacob's name was changed to Israel), then Gentiles now attached to Christ were no longer acknowledged as "Gentiles" in a spiritual sense. They had now become "the Israel of God" (Galatians 6:16).
"For you are all the children of God by faith in Christ Jesus. For as many of you as have been baptized into Christ have put on Christ. There is neither Jew nor Greek, there is neither bond nor free, there is neither male nor female: for you are all one in Christ Jesus. And if you be Christ's, then are you Abraham's seed, and heirs [along with Israelites] according to the promise." Galatians 3:26–29
Similarly, Paul said in Romans 11:13–36 that the Gentiles being "in Christ" (and Christ was an Israelite in whom there was no guile and a legitimate son of Abraham) made them also to be children of Abraham and Israel as was Christ or any natural born Israelite. And since it was clearly understood by Paul that "all Israel shall be saved" (Romans 11:26), this of necessity included the Gentiles who accepted Christ and were grafted into Israel. Thus, these Gentile converts were no longer reckoned as Gentiles. They were now "the Israel of God" (Galatians 6:16).
This was how the Gentiles could become partakers of all promises and covenants that pertained to the Israelites. This grafting into the stock of Israel made them able to be heirs of the New Covenant relationship that God was making only with Israel.
A Major Problem Developed with this Concept
While this spiritual teaching that Gentiles were now "in Christ" denominated them as being Israel along with the other Israelites, a question quickly arose concerning the required conduct of these new Israelites. Should they be circumcised as were ordinary Israelites? Should they keep the days, times, seasons, and years that all Jews observed and rejoiced in? Should these new "Israelites" be required to eat the clean foods found in Leviticus chapter 11?
Some Christian authorities, especially those in Jerusalem, felt that if the Gentiles were now going to be acknowledged as "Israelites," then it was only common sense they should adopt the same customs as the Jews. This included circumcision, Sabbath, and holyday keeping, the clean food laws, etc. This matter had to be worked out and we find much argument about it in the Books of Acts and Galatians (along with Second Corinthians chapters 11 and 12). In other words, should the Gentiles (now "Israelites") keep the Law of God revealed in the Old Testament and sustained by Christ while teaching in the flesh, or could the Gentiles be given some kind of special relationship different from ordinary Jews? This was the dilemma. What laws and customs should the Gentiles (now "Israelites") observe?
Remember the agreement the Jerusalem Church and Paul came to?
(Info taken from ELM)