Acts chapter 15 is a chapter about the meaning of Christian faith. It is perhaps the most important chapter in the New Testament outside of the gospels. The books of Romans, Galatians, and Hebrews are based mostly on the decisions and statements made in Acts 15.
The leaders of the early church were all Jewish converts, and by Acts 15 they were dealing with a crisis of faith as many Gentile converts were attempting to join the Christians. The leaders met in Jerusalem to discuss how to handle Gentiles who professed Jesus as Lord and Savior.
There were many in favor of making Gentiles get circumcised before they could call themselves Christians. Some wanted to go even farther and make them essentially become Jewish before they could become Christian. Others determined that Jesus had died for the sins of the world, and therefore his purpose was to save from sin, not to bring gentiles into Judaism.
The core of this question was a question of what the primary problem with humanity is.
To Jews, the big problem was that that so many people failed or outright refused to read the Old Testament and practice the teachings of the law. If everyone obeyed Genesis through Deuteronomy and paid attention to the prophets, then God’s kingdom would come to earth and there would be joy. This was the belief of the Pharisees and the vast majority of people in Israel. They expected God’s kingdom to come when they all got serious about obeying God’s commands.
To Gentile converts the biggest problem was that they had no access to God because they were sinful and he was perfect. They could not reach God because they were unworthy. Jesus solved this problem by taking their sin and imputing righteousness to them.
In Acts 15 the Jewish Christian leaders decided unanimously that Jesus had died for our sins, in order to save us, not so that we could be circumcised, but so that our sins would be washed away.
It may seem obvious now, but it was a meeting of extreme importance, and the outcome of the meeting ensured that true Christian faith and practice would be taught.
We owe a great deal to Peter, James, and the other leaders who agreed together in Acts 15.