Theophylact of Ohrid

"You sit to judge according to the law, and, contrary to the law, you order me to be struck." That is, transgressing the law, becoming guilty of lawlessness; as if to say: "Being worthy of very many punishments." "You whitewashed wall!" He calls him this because although Ananias assumed the bright appearance of a man who supposedly defends the law and judges according to the law, his mind was full of lawlessness. Therefore Paul also exposes in him the hypocritical appearance of outward devotion to the law.

Commentary on Acts ↗

Oecumenius

"God will strike you, you who have been puffed up." It is worth asking how the same speaker elsewhere says, "When condemned we bless, when persecuted we endure," (1 Cor. 4:12) whereas here, on the contrary, he not only condemns but also curses. And we say that these expressions are more a matter of boldness than of anger. For he did not wish to appear scornful toward the centurion. …

Commentary on Acts ↗

Augustine of Hippo

Of course, those who do not understand him think that [Paul] uttered a reproach when he had been slapped by order of the high priest, for, with seeming insolence, he then said, "God will strike you, you whitewashed wall." But those who understand him take this as a prophecy. The "whitewashed wall" stands for hypocrisy; it is pretense, veiled beneath the priestly dignity, and under this title—as though beneath a white covering—it conceals, as it were, an inner slimy filthiness.

Sermon On The Mount 1.19.58 ↗

Bede

God will strike you, whitewashed wall. He did not say this disturbed in mind, but indeed he spoke prophetically, because that figurative priesthood, which was composed in the likeness of a whitewashed wall, was to be struck and destroyed when the true priesthood of Christ had come, with the apostles preaching the Gospel. And that is why he said: God will strike you. He did not say, may He strike you: indeed indicating this in the indicative mode that it would happen, not cursing in the optative. …

Commentary on Acts ↗