5 But there rose up certain of the sect of the Pharisees which believed, saying, That it was needful to circumcise them, and to command them to keep the law of Moses.
He then cursorily touches on his own conversion from a persecutor to an apostle-confirming thereby the Acts of the Apostles, in which book may be found the very subject of this epistle, how that certain persons interposed, and said that men ought to be circumcised, and that the law of Moses was to be observed; and how the apostles, when consulted, determined, by the authority of the Holy Ghost, that "a yoke should not be put upon men's necks which their fathers even had not been able to bear." Now, since the Acts of the Apostles thus agree with Paul, it becomes apparent why you reject them.
Great effrontery this, of the Pharisees, that even after faith they set up the Law, and will not obey the Apostles. But see these, how mildly they speak, and not in the tone of authority: such words are amiable, and more apt to fix themselves in the mind. Observe, it is nowhere a display of words, but demonstration by facts, by the Spirit. And yet, though they have such proofs, they still speak gently. But even if they would needs bring over the Gentiles to their side, they learn that neither must the Apostles overlook it.
Saying that it is necessary to circumcise them. The Galatians were deceived by this error, believing that circumcision and the ceremonies of the law should be mixed with the grace of baptism.
[AD 220] Tertullian on Acts 15:5