15 Neither [in that case] would they have had such a tenor with regard to the first covenant, as not even to have been willing to eat with the Gentiles. For even Peter, although he had been sent to instruct them, and had been constrained by a vision to that effect, spoke nevertheless with not a little hesitation, saying to them: “You know how it is an unlawful thing for a man that is a Jew to keep company with, or to come unto, one of another nation; but God has shown me that I should not call any man common or unclean.
Therefore came I without gainsaying;” indicating by these words, that he would not have come to them unless he had been commanded. Neither, for a like reason, would he have given them baptism so readily, had he not heard them prophesying when the Holy Ghost rested upon them. And therefore did he exclaim, “Can any man forbid water, that these should not be baptized, who have received the Holy Ghost as well as we?” He persuaded, at the same time, those that were with him, and pointed out that, unless the Holy Ghost had rested upon them, there might have been some one who would have raised objections to their baptism.
And the apostles who were with James allowed the Gentiles to act freely, yielding us up to the Spirit of God. But they themselves, while knowing the same God, continued in the ancient observances; so that even Peter, fearing also lest he might incur their reproof, although formerly eating with the Gentiles, because of the vision, and of the Spirit who had rested upon them, yet, when certain persons came from James, withdrew himself, and did not eat with them. And Paul said that Barnabas likewise did the same thing. Thus did the apostles, whom the Lord made witnesses of every action and of every doctrine— for upon all occasions do we find Peter, and James, and John present with Him— scrupulously act according to the dispensation of the Mosaic law, showing that it was from one and the same God; which they certainly never would have done, as I have already said, if they had learned from the Lord [that there existed] another Father besides Him who appointed the dispensation of the law.
Source: Adversus haereses (New Advent)