John Chrysostom

In this respect we most marvel at Mark, because not only did he refuse to hide Peter's fault, but wrote the account of it in greater detail than the others. And it is for this very reason that he is called Peter's disciple.

The Gospel Of St Matthew 85.1 ↗

Theophylact of Ohrid

But repentance and tears restored him again to Christ. For it says: "he began to weep." That is, covering his face, he began to weep bitterly. Matthew (speaking of Peter's denial) said unclearly: "before the rooster crows" (Mt. 26:34); Mark clarified this: "before the rooster crows twice." Roosters usually make several calls at one time, then fall asleep, and after some time begin crowing again. …

Commentary on Mark ↗

Augustine of Hippo

As holy David repented for his deadly crimes and still retained his kingship, so the blessed Peter earnestly repented, having denied the Lord, and shed such bitter tears, yet remained an apostle.

Letter 185, To Boniface ↗

Gregory the Dialogist

In this matter we must consider why Almighty God permitted him whom He had determined to place over the whole Church to fear the voice of a servant girl and to deny himself. This we recognize was done by a dispensation of great mercy, so that he who was to be the Pastor of the Church might learn through his own fault how he ought to show mercy to others. Therefore He first showed him to himself, and then set him over the rest, so that from his own weakness he might learn how mercifully he should bear with the weaknesses of others.

40 Homilies on the Gospels, Homily 21 ↗

Bede

And Peter remembered the word that Jesus had said to him: "Before the rooster crows twice, you will deny me three times." And he began to weep. How harmful are the colloquies of the wicked? Peter himself among the faithless denied even knowing a man, whom among his fellow disciples he had already confessed to be the Son of God. But neither could he, held in the courtyard of Caiaphas, perform penitence. …

On the Gospel of Mark ↗