John Chrysostom

But if while preaching the Jewish doctrines, he suffered thus, had he preached the doctrines of the glory of Christ, how would they have endured him? While "purifying himself" he was intolerable, and how should he have been tolerable while preaching? What lay ye to his charge? What have ye heard? He spoke nothing of the kind. He was simply seen, and he exasperated all against him. Well might he then be set apart for the Gentiles: well might he be sent afar off: there also destined to discourse to the Gentiles.

Homily on Acts 55 ↗

Theophylact of Ohrid

Paul had his hair cut and fulfilled all the Jewish rites, but not because he changed his opinion (for that would have been a thing impure from vice), but because love is condescending. "He declared the completion of the days of purification," that is, he himself declared about himself that the purification was completed.

Commentary on Acts ↗

Oecumenius

And to do this in Jerusalem is attainable. Do this then here, so that it may be possible for you to do that abroad. Then he was persuaded by them, and he shaved himself and fulfilled all the Jewish rites. [AMMONIUS]

Commentary on Acts ↗