9 And recognizing the grace that I had been given, James, Cephas, and John—those reputed to be pillars—gave me and Barnabas the right hand of fellowship, so that we should go to the Gentiles, and they to the circumcised. -Galatians 2:9 BSB
The word Cephas in Aramaic means rock and it is translated in Greek as Peter (Petros), see John 1:42. Peter was a disciple of Jesus Christ and the brother of Andrew. Satan with Jesus’ permission attacked and caused Peter to deny Jesus three times. Afterward, Peter repented and was strengthened by the Lord. Peter wrote 1st & 2nd Peter.
John was one of Jesus’ disciples who wrote the Gospel of John, 1st, 2nd, and 3rd John, and the Book of Revelation. He was the only one that didn’t get martyred for his faith in Jesus Christ. However, he did get persecuted, see Revelation 1:9, as he was banished to the Island of Patmos for his faith, where he wrote Revelation through visions given to him by an angel from the Lord, see Revelation 1:1.
These three men were the ones who held up the faith strongly and helped lead the churches in sound doctrine according to the teachings of the Lord Jesus Christ (found in the New Testament). They followed the principles of the Old Testament, understanding them through the lens of the Messiah as taught by Him.
James, Cephas (Peter), and John understood that Paul and Barnabas were to go out and make disciples in the Gentile world to fulfill what Jesus said in Matthew 28:18-20, which says…
18 Then Jesus came to them and said, “All authority in heaven and on earth has been given to Me. 19 Therefore go and make disciples of all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Spirit, 20 and teaching them to obey all that I have commanded you. And surely I am with you always, even to the end of the age.” -Matthew 28:18-20 BSB
Some of the apostles went to their own people with the gospel while the rest, including Paul and Barnabas, went to the Gentiles, see verse 8 and its commentary.