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.
By revelation. Not to satisfy doubts of my commission, but by divine "revelation," which showed him more than the Church knew of the import of the crisis. Quite consistent with his also being a deputy from the church of Antioch, as Acts 15:2 states. He by this revelation was led to suggest the sending of the deputation. So Peter was led by vision, and at the same time by Cornelius' messengers, to go to Caesarea, (Acts 10:1-48.)
I ... communicated unto them - to the Christians at Jerusalem, the Gospel in general, as I preached it.
But privately - that he and they might decide previously on the principles to be set forward before the public council. The Jerusalem apostles should know beforehand that the particular doctrine Paul preached to the Gentiles was the same as theirs, and had received divine confirmation in the results worked on the Gentiles. He and Barnabas related to the multitude, not the details of doctrine they preached (as Paul did privately to the apostles), but only the general facts and the miracles vouchsafed in proof of God's sanctioning their preaching to the Gentiles (Acts 15:2; Acts 15:4; Acts 15:12).
To them which were of reputation - James, Cephas, and John, and probably some of the "elders" (Galatians 2:6; Galatians 2:9). lest, etc. - i:e., that they might see that I am not running, and have not run, in vain. Paul does not himself fear lest he be running, or had run, in vain; but lest he should, if he gave them no explanation, seem so to them. His race was the swift-running proclamation of the Gospel to the Gentiles (cf. "Word ... have free course" - literally, "run," 2 Thessalonians 3:1). His running would have been in vain, had circumcision been necessary, since he did not require it of big converts.