Why did the Lord tell Paul to go up to Jerusalem and confront the Twelve?
The Lord told Paul by revelation to go to Jerusalem and confront the Twelve so that the apostles could settle the debate over whether salvation requires keeping the Law of Moses and circumcision. Settling this was necessary for the gospel to move forward into the Gentile world without legalistic requirements.
Les explains that Paul says in Galatians 2, “I went up by revelation.” When Paul uses “revelation,” it means the Lord spoke it. The Lord evidently told Paul it was time to get up to Jerusalem and confront the Twelve. They had to settle the issue of whether a person is saved by faith alone or must also keep the Law of Moses and practice circumcision. If they did not settle it, they could never move out into the Gentile world. This was about 51 AD, some 22 years after Pentecost, showing the debate had persisted a long time.
And I went up by revelation, and communicated unto them that gospel which I preach among the Gentiles, but privately to them which were of reputation, lest by any means I should run, or had run, in vain.