Jerome

Then he began to curse and swear, saying that he did not know the man. And immediately the rooster crowed. And Peter remembered the word of Jesus that he had said: Before the rooster crows, you will deny me three times. In another Gospel, we read that after Peter's denial and the rooster's crowing, the Savior looked at Peter and with his gaze, he caused him to burst into bitter tears. It was not possible for him to remain in the darkness of denial, whom the light of the world had looked upon. …

Commentary on Matthew ↗

Thomas Aquinas

Then he began to curse and to swear etc. There are some who wish to excuse Peter, saying that he did not sin; hence when he said I know not the man, it is true, a man, but a man and God. And this is not good, because it imputes a lie to Christ: for Christ had said thou shalt deny me. Therefore it is better to say that Peter lied rather than Christ. Likewise, it should be noted that he not only denied Christ but denied being a Christian. Hence in one denial he said I know him not, namely, I am not a Christian. …

Commentary on Matthew ↗

Rabanus Maurus

After the third denial comes the cock-crow; by which we may understand a Doctor of the Church who with chiding rouses the slumbering, saying, Awake, ye righteous, and sin not. (1 Cor. 15:14.) Thus Holy Scripture uses to denote the merit of divers cases by fixed periods, as Peter sinned at midnight and repented at cock-crow.

Catena Aurea by Aquinas ↗