John Chrysostom

Great was the zeal of Peter, who fled not when He saw the others fly, but remained, and entered in. For though John also went in, yet he was known to the Chief Priest. He followed afar off, because he was about to deny his Lord.

Catena Aurea by Aquinas ↗

Theophylact of Ohrid

Peter, who was more fervently devoted to the Master, followed at a distance. If John also followed, it was not as a disciple but as an acquaintance of the high priest (Jn. 18:15).

Commentary on Matthew ↗

Augustine of Hippo

And also that the Church should follow, i. e. imitate, the Lord's Passion, but with great difference. For the Church suffers for itself, but Christ for the Church.

Catena Aurea by Aquinas ↗

Jerome

And entering inside, he sat with the ministers, to see the end.... Whether out of love for his disciples or out of human curiosity, the high priest wanted to know what he would decide about the Lord: whether he would condemn him to death or release him after beating him. And there was a difference of opinion among the ten apostles and Peter. Those (people) flee, but this one, although far away, still follows the Savior.

Commentary on Matthew ↗

Thomas Aquinas

But Peter followed him afar off. The place has been treated; here he treats of Peter arriving. First, he is led; then Peter arrives. And he does three things: because first, he touches on the manner; secondly, how he arrived while following. That he arrived, this was of fervor; that from afar, this was of fear; hence it was signified that the Church, founded on Peter's faith, would follow Christ, yet from afar; because Christ suffered for the Church, not for himself; but Peter and the Church suffered for themselves. …

Commentary on Matthew ↗