Galatians 2:1-5
Then after fourteen years I went up again to Jerusalem with Barnabas, taking Titus along with me. 2 I went up because of a revelation and set before them (though privately before those who seemed influential) the gospel that I proclaim among the Gentiles, in order to make sure I was not running or had not run in vain. 3 But even Titus, who was with me, was not forced to be circumcised, though he was a Greek. 4 Yet because of false brothers secretly brought in—who slipped in to spy out our freedom that we have in Christ Jesus, so that they might bring us into slavery— 5 to them we did not yield in submission even for a moment, so that the truth of the gospel might be preserved for you.
The point about the reference to Titus is this: If the apostles, Peter, James and the others, had been in agreement with these Jewish detractors who were subverting the truth in Galatia, claiming that what they were teaching about circumcision was in harmony with what Peter and the others preached, then Titus must surely have been compelled to be circumcised. But, says Paul, Peter did not in fact demand his circumcision, for the simple reason that Peter did not consider circumcision to be necessary for salvation. But if Peter had insisted, Paul would have resisted to his last breath, as we may gather from what he says in 5. Thus ardently did he contend for the faith once delivered to the saints (Jude 3), thus firmly was he set for the defence of the gospel (Philippians 1:7, 17). We have already underlined the centralities of the gospel as unfolded in this epistle: justification by faith (1:4), identification and union with Christ (2:20), liberty in Christ (5:1) and the fruit of the Spirit (5:22, 23). We could with equal force underline issues such as: the authority of Scripture, the reality of the eternal world, heaven and hell, the necessity of regeneration and conversion, and the expectation of the return of Christ; for these doctrines also are comprehended in the phrase 'the truth of the gospel'. They are certainly not matters on which Paul would yield in any way or against which he would countenance an attack.