Theophylact of Bulgaria
Therefore Peter was seized with fear, and for-getting the word of the Lord, which said, Whosoever shall confess me before men, him will I confess before my Father, (Matt. 30, 32) he denied our Lord; wherefore there follows: But he began to curse and to swear, saying, I know not this man of whom ye speak.
For tears brought Peter by penitence to Christ. Confounded then be the Novatians, who say that he who sins after receiving baptism, is not received to the remission of his sin. For behold Peter, who had also received the Body and Blood of the Lord, is received by penitence; for the failings of saints are written, that if we fall by want of caution, we also may be able to run back through their example, and hope to be relieved by penitence.
The Lord allowed this to happen to him by His providence, that is, lest he should be too much elated, and at the same time, that he might prove himself merciful to sinners, as knowing from himself the result of human weakness. There follows: And he went out into the porch; and the cock crew.