But with regard to the countenance of Peter and the rest of the apostles, he tells us that "fourteen years after he went up to Jerusalem," in order to confer with them about the rule which he followed in his gospel, lest perchance he should all those years have been running, and be running still, in vain, (which would be the case, ) of course, if his preaching of the gospel fell short of their method.
These men he had as witnesses, through whom he proved that his gospel was given to him through revelation, seeing that he said “Barnabas went up with me,” and he also took Titus, whose faith and gospel were approved by everyone.
His renown had been growing for a long time among all the Jews, though he had not been seen face to face … but on account of the law he had acquired a bad reputation among the Jews, as though his preaching was out of harmony with the preaching of the other apostles. Many were having doubts on account of this, which were sufficient to make the Gentiles anxious, in case they had been trained in something other than that which was preached by the apostles who had been with the Lord. For on this precise occasion the Galatians were undermined by Jews who were saying that Paul taught something other than Peter taught. This is the reason for his going up to Jerusalem, at the bidding of the Lord’s revelation, disclosing to them the implications of his preaching, with Barnabas and Titus as witnesses of his preaching, one from the Jews and one from the Gentiles, so that if any took offense at him it might be assuaged by their testimony.
(Chapter 2 - Verses 1, 2) Then after fourteen years, I went up again to Jerusalem with Barnabas, taking Titus along also. I went up by revelation, and I discussed with them the gospel that I preach to the Gentiles, but privately with those who seemed to be influential, so that I might not be running in vain or have run in vain. This matter arose 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. And to speak more truthfully, the Greek word ἀνεθέμην conveys something different than what is understood among us, namely that we compare what we know with a friend and place it in their lap and conscience, so that, with equal counsel, what we know may either be approved or disapproved. Therefore, after fourteen years, he went up to Jerusalem. And he who had previously gone only to see Peter and stayed with him for fifteen days, now says that he went there to confer with the apostles about the Gospel. And he took along Barnabas, who was circumcised, and Titus, who was uncircumcised among the Gentiles, so that every word may be established by the testimony of two or three witnesses (Deut. 19:15). But it is one thing to compare, another to learn. Among those who compare, there is equality; between the teacher and the learner, the one who learns is lesser. At the beginning of faith, he saw the apostles during their journey. After seventeen years (as he himself says), he speaks fully with them and humbles himself: and lest he might be running or might have run in vain, he inquires. For a twofold reason, so that the humility of Paul, who, as a teacher, had already surpassed the apostles who came before him in the whole world of the Gentiles, might be shown: and so that the Galatians may not say that they rejected his Gospel, also condemning those who presided over the Churches in Judea. Moreover, it also teaches that for the sake of the faith of Christ and the liberty of the Gospel, he dared to lead Titus, an uncircumcised man, to those same individuals who knew more about him, who accused him of breaking the Law, destroying Moses, and completely doing away with circumcision. And in the midst of such a great multitude of Jews and his enemies, who desired to shed his blood out of zeal for the Law, neither he nor Titus gave way to fear, but stood firm in necessity. They were able to obtain forgiveness, either in terms of position or authority of the elders, or in terms of the number of churches that believed in Christ from the Jews, or in terms of time, so as not to endure such great envy at the same time. Some say that after fourteen years he went up to Jerusalem, when a dispute arose among the believers in Antioch regarding the observance or omission of the Law, as recorded in the Acts of the Apostles. It was decided to go to Jerusalem and wait for the judgment of the elders, when Paul and Barnabas themselves were sent. And this is what is written in the Latin codices: We yielded to subjection for a time, so that the truth of the Gospel would persist among you. That is why, clearly, Paul and Barnabas were sent to Jerusalem as if it were doubtful, in order to confirm by the judgment of the elders that the grace of the Gospel had also been given to the believers, and that there would be no further doubt about the omission of circumcision. For it had been commanded by the letters of the apostles that the yoke of the Law should be removed from those who believed in Christ from the Gentiles. However, it can be understood that when he says, 'I conferred with them the Gospel that I preach to the Gentiles,' he means separately to those who seemed important, so that I might not run or have run in vain, and thus it can be understood that he shared with the apostles in secret the grace of Gospel freedom and the abolishment of the old Law, because of the multitude of believing Jews who were not yet able to hear about the fulfillment and end of the Law, and who, in the absence of Paul, had falsely boasted in Jerusalem that he was going on a pointless or fruitless journey, thinking that the old Law should not be followed. Not because Paul feared that he had preached a false Gospel among the Gentiles for seventeen years; but to show his predecessors that he was not running or had run in vain, as they had thought in their ignorance.
Both [Barnabas and Titus] spent an extended time in Antioch, making a large body of converts and binding them to live according to the law of grace. But some supporters of the law who arrived from Judea tried to persuade the Gentiles to adopt the way of life according to the law But those great heralds of the truth, Paul and Barnabas, repudiated the teaching they promoted. They wished to persuade the congregation of the faithful that this was also the view of the great apostles. So they immediately went straightway to Judea, to apprise the apostles of what was going on.
[AD 202] Irenaeus on Galatians 2:1