This is a fair question. First it was prophesied in the Old testament that God would make a new covenant with man.
Jer 31:31-34 KJV Behold, the days come, saith the LORD, that I will make a new covenant with the house of Israel, and with the house of Judah: (32) 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 brake, although I was an husband unto them, saith the LORD: (33) But this shall be the covenant that I will make with the house of Israel; After those days, saith 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. (34) And they shall teach no more every man his neighbour, 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, saith the LORD: for I will forgive their iniquity, and I will remember their sin no more.
This was addressed in the New testament in two areas first here.
Heb 8:6-13 KJV But now hath he obtained a more excellent ministry, by how much also he is the mediator of a better covenant, which was established upon better promises. (7) For if that first covenant had been faultless, then should no place have been sought for the second. (8) For finding fault with them, he saith, Behold, the days come, saith the Lord, when I will make a new covenant with the house of Israel and with the house of Judah: (9) Not according to the covenant that I made with their fathers in the day when I took them by the hand to lead them out of the land of Egypt; because they continued not in my covenant, and I regarded them not, saith the Lord. (10) For this is the covenant that I will make with the house of Israel after those days, saith the Lord; I will put my laws into their mind, and write them in their hearts: and I will be to them a God, and they shall be to me a people: (11) And they shall not teach every man his neighbour, and every man his brother, saying, Know the Lord: for all shall know me, from the least to the greatest. (12) For I will be merciful to their unrighteousness, and their sins and their iniquities will I remember no more. (13) In that he saith, A new covenant, he hath made the first old. Now that which decayeth and waxeth old is ready to vanish away.
And also here:
Act 15:18-22 KJV 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. (21) For Moses of old time hath in every city them that preach him, being read in the synagogues every sabbath day. (22) Then pleased it the apostles and elders, with the whole church, to send chosen men of their own company to Antioch with Paul and Barnabas; namely, Judas surnamed Barsabas, and Silas, chief men among the brethren:
James The brother of Jesus was the head of the early church there was a big question on what to do with the gentiles that came into the church and this was the decision on what to do. There were some that wanted the gentiles to get circumcised and follow the dietary laws. This was the only part of the old testament law that was placed on them to keep. The Jewish believers still had to keep the law of the old testament.