It is ridiculous and Judaizing (Gal. 2:14) to think I as a Gentile, not a Jew, was ever under the Old Covenant, the Mosaic Law.
Christ set a sinless example of how to walk in obedience to the Mosaic Law, so Paul's problem with the Judaizers was not that they were teaching Gentiles how to follow Christ, but that they were wanting to require Gentiles to become circumcised in order to become saved (Acts 15:1), which I have never suggested, so I am in complete agreement with Paul's stance against the Judaizers. The reason that God commanded circumcision was never in order to become saved, so circumcision was being used for a man-made purpose that went above and beyond the purpose for which God commanded it. So the Jerusalem Council upheld the Mosaic Law by correctly ruling against that requirement, and a ruling against requiring something that God never commanded should not be mistaken as being a ruling against obeying what God has commanded, as if the Jerusalem Council had the authority to countermand God.
In Acts 10:28, Peter referred to a law that forbade Jews to visit or associate with Gentiles, however, this is not a law that is found anywhere in the Mosaic Law, so again it is a man-made law. It was this law that Peter was obeying in Galatian 2:11-16 when he stopped visiting or associating with the Gentiles, and by doing so, he was giving credibility to those who were wanting to requiring Gentiles to obey their works of the law in order to become justified, which is why Paul rebuked him and reiterated that we are justified by faith, not by works of the law, so again you should be more careful not take mistake something that was only said against obeying man-made requirements as being against obeying God.
While we are under the New Covenant and not the Mosaic Covenant, we are nevertheless still under the same God with the same nature and therefore the same laws for how to testify about His nature. The existence of sin requires there to be a standard of what is and is not sin, and that standard is God's nature, which has been revealed through His law. If Gentiles were not under the God's law, then Gentiles would have no need to refrain from sin and would have no need for Jesus to save them by giving himself to redeem them from all lawlessness. However, God is sovereign, so we are all under His law and are obligated to obey it regardless of which covenant we are under, if any, such as when God judged the world with the Flood because of their wickedness. They didn't get the choice of whether they wanted to be under God's law and neither do you, but the choice you do get to make is whether or not you are going to heed the Gospel message, repent, and obey.
Acts 15 makes it perfectly plain I was not ever to be put under that yoke, which even the Jews could not bear!
In Acts 15:1, Judaizers who had come from Judea wanted to require all Gentiles to become circumcised according to the custom of Moses in order to be saved, which was a false position of being saved by works. They were countered in Acts 15:4-5 by a group of believers who rose up from the Pharisees that were inside of Jerusalem, who said that it is necessary for Gentiles to become circumcised and order them to keep the Torah, so because they were valid believers in the faith, they were taking the position of salvation by faith and that Gentiles needed to obey the Torah as a matter of obedience as the expression of their faith. The third group was the Jerusalem Council in Acts 15:4, who was facilitating the debate between the first two groups, and the fourth group were Gentile converts who were new to the faith, who were still influenced by pagan practices. So no one was arguing for the position that Gentiles didn't need to obey the Torah.
In Acts 15:6-9, Peter made the case that God who knows their hearts acknowledged them by giving them the Holy Spirit and purified their hearts by faith, so he was supporting the second group. In Acts 15:10, the burden that no one could bear was salvation by works in Acts 15:1. God said in Deuteronomy 30:11-14 that the Torah is not too difficult to obey, and in 1 John 5:3, the commandments of God are not burdensome. In Acts 15:11, Peter reiterated that we are saved by grace, which again counted the first group and supported the second, which Paul and Barnabas gave additional support to in verse 12. In Acts 15:13-18, James gave a third witness by quoting the prophets by showing that it was always God's plan to include Gentiles in the restoration of Israel. So they all ruled against salvation by works and supported the position presented in verse 5, while no one made a case against Gentiles keeping the Torah.
Not only was it unreasonable to require obedience to the Torah to earn salvation, but it was also unreasonable to expect new believers to understand and apply all of God's laws on day one. All of God's works are known to Him from eternity, but Gentiles are still learning them, so James was softening the position taken in verse 5. The four laws listed in Acts 15:19-21 are all from the Torah, so again, no one was arguing against Gentiles obeying it, but rather James was saying that they needed to first focus their priority on refraining from pagan idolatry, and then continue to learn how to obey Moses by hearing him taught every Sabbath in the synagogues, which was in support of the position in verse 5. Those four laws are clearly not an exhaustive list of everything that a mature Gentile believer is required to do.
Adapted from John Trapp's comments on Matthew 5:17 -
But to fulfil it] To complete and accomplish it (πληρωσαι), for he fulfilled all righteousness, and finished the work that was given him to do, "I glorified thee on the earth, having accomplished the work which thou hast given me to do." (John 17:4). A new commandment also gave be unto us, that we love one another; which love is the complement of the law and the supplement of the gospel. Besides, "Christ is the end of the law to every one that believeth," (Rom. 10:4); yea, he doth all his works in us, and for us, saith the Church, "LORD, thou wilt ordain peace for us: for thou hast also wrought all our works for us. (Isa 26:12). Thus Christ still fulfils the law in his people; into whose hearts he putteth a disposition answerable to the outward law in all things, as in the wax is the same impression that was upon the seal. This is called the law of the mind, "I thank God through Jesus Christ our Lord. So then I myself with the mind serve the law of God; but with the flesh the law of sin. (Rom 7:25), and answereth the law of God without, as lead answers the mould, as tally answereth tally, as indenture indenture, "For finding fault with them, he saith, 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; Not according to the covenant that I made with their fathers In the day that I took them by the hand to lead them forth out of the land of Egypt; For they continued not in my covenant, And I regarded them not, saith the Lord. 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 on their heart also will I write them: And I will be to them a God, And they shall be to me a people:" (Heb 8:8-10)
"Do not suppose that I have come to abolish the law and the prophets; I did not come to abolish, but to complete." (Matt 5:17 REB)
"Do not for a moment suppose that I have come to abrogate the Law or the Prophets: I have not come to abrogate them but to give them their completion." (Matt 5:17 Weymouth)
In Matthew 5:17-19, Jesus said that he came to fulfill the law in contrast with saying that he came not to abolish it, so fulfilling the law should not be interpreted as meaning essentially the same thing as abolishing them. Furthermore, I quoted the only definition the NAS Greek lexicon lists as being specifically in regard to the law, so when that definition fits with what Jesus immediately proceeded to do next after he said he came to fulfill the law, then you should give a reason for why you have rejected it, though completing our understanding of how to correctly obey the law also fits. Again, in Galatians 5:14, fulfilling the law refers to something that countless people has done, not to something that Jesus did on our behalf, and fulfilling the Law of Moses should consistently be interpreted in the same way as fulfilling the Law of Christ, which is again something that anyone can do.
The command to love one another is found in Leviticus 19:18. In Matthew 7:23, Jesus said that he would tell those who are workers of lawlessness to depart from him because he never knew them, so a relationship with Christ is the goal of the Mosaic Law, which is the same thing that Romans 10:4 is speaking about. The Mosaic Law is the law that Paul served with his mind, and in Hebrews 8:10, the New Covenant still involves following the Mosaic Law.
"I marvel that ye are so quickly removing from him that called you in the grace of Christ unto a different gospel; which is not another gospel: only there are some that trouble you, and would pervert the gospel of Christ. But though we, or an angel from heaven, should preach unto you any gospel other than that which we preached unto you, let him be anathema." (Gal 1:6-8)
Jesus began his ministry with the Gospel message to repent for the Kingdom of God is at hand (Matthew 4:17-23), and the Mosaic Law was how his audience knew what sin is, so repenting from our disobedience to it is an integral part of the Gospel of the Kingdom, which Jesus prophesied would be proclaimed to the nations (Matthew 24:12-14). The same goes for how Peter's audience knew what sin is when he told them to repent for the forgiveness of sin in Acts 2:38. In Romans 15:4, Paul said OT writings are for our instruction and in 15:18-19, his Gospel message involved bringing the Gentiles to full obedience in word and in deed, so his Gospel was on the same page in regard to teaching repentance from our sins.