John Chrysostom
They advise him to justify himself not by words, but by deed. “So that they may shave their heads,” they say, “and all will know that what they have heard about you is not true, but that you yourself also continue to keep the law.”
They did not say, “you teach,” but, “what they have heard about you is not true,” again expressing that they themselves are convinced of this.
“And that you yourself also continue,” that is, that besides this you yourself also keep the law.
It was necessary to know not only whether he taught others, but also whether he himself observed it. Paul does not resist this, but obeys.
From this it is clear that this did not have to be done absolutely, for they therefore persuade him. Rather, it was a matter of prudence and condescension.
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This was no obstacle to the preaching, because they themselves had given the command concerning the Gentiles.
For this reason Paul himself, while arranging such a matter, later rebukes Peter. He does this not without reason, since what Paul did here, Peter did there while hiding and holding to his own opinion (Gal 2:11-14).
They did not say that Jews must not be taught in the same way as Gentiles, or that one may even refrain from preaching to them. Rather, they said that something more must be done.
It had to be done so that they might be assured that you keep the law. This is a matter of condescension; do not fear.
And see: they did not persuade him before first explaining that prudence and benefit required this. In Jerusalem, they say, it is permitted to do this.
Therefore do it here, so that you may be able to do the other thing outside Jerusalem.