Why did Paul and Barnabas go up to Jerusalem to confront Peter?
Paul and Barnabas went up to Jerusalem because Judaizers were troubling the Gentile churches, claiming converts could not be saved unless they kept the Law of Moses and practiced circumcision. After many years the issue came to a head, so by the Lord's direction they went to confront Peter and the Jerusalem hierarchy.
Les explains that the church at Antioch in Syria had become predominantly Gentile, where Paul and Barnabas were laboring and from which they moved out into the Gentile world of Asia Minor and Greece. Everywhere they went, their little congregations were constantly subjected to Judaizers who tried to bring them back under the Law, claiming that Paul's converts could not be saved unless they kept the Law of Moses and practiced circumcision. Finally, after many years, this came to a head, and by the Lord's direction Paul and Barnabas were sent up to Jerusalem to confront Peter and the hierarchy there. Paul rehearses this in Galatians chapter 2, and Luke's account of the same meeting is later picked up in Acts chapter 15. The reference to those who 'seemed to be somewhat' implies they were not what they thought they were.