27 As, when we read in the Gospel, “You have received a blow in the face, make ready the other cheek.” Now as an example of patience can none be found than that of the Lord Himself more potent and excellent; but He, when smitten on the cheek, said not, Behold here is the other cheek, but He said, “If I have spoken ill, bear witness of the evil; but if well, why do you smite Me?” Where He shows that the preparation of the other cheek is to be done in the heart. Which also the Apostle Paul knew, for he, too, when he was smitten on the face before the high priest, did not say, Smite the other cheek: but, “God,” says he, “shall smite you, you whited wall: and do you sit to judge me according to law, and contrary to law commandest me to be smitten?” with most deep insight beholding that the priesthood of the Jews was already become such, that in name it outwardly was clean and fair, but within was foul with muddy lusts; which priesthood he saw in spirit to be ready to pass away through vengeance of the Lord, when he spoke those words: but yet he had his heart ready not only to receive other blows on the cheek, but also to suffer for the truth any torments whatever, with love of them from whom he should suffer the same.
Source: On Lying (New Advent)