During the first century, there were some jewish christians who taught that a gentile could not become a christian unless the gentile first became jewish and was circumcised. They claimed that the brethren at Jerusalem, including those Apostles who lived there had taught them this. Paul went to Jerusalem to find out if it were true that this was being taught by the brethren there

In Acts 15, Peter arose and told the history of God having COMMANDED him to preach to the gentiles at the household of Cornelius.

After Peter finished, Paul and Barnabas gave EXAMPLES of God's approval of their having preached to the Gentiles.

Then James, used passages from the Old Testament to make a NECESSARY INFERENCE.

The conclusion was that God had authorized that the gospel be preached to gentiles, who could become christians without circumcision.