John 18:15

15.And another disciple. Some have been led astray, by a slight conjecture, to suppose that this disciple was the Evangelist (139) John, because he is accustomed to speak of himself without mentioning his name. But what intimacy with a proud high priest could John have, who was a mean fisherman? And how was it possible for him, being one of Christ’s household, to be in the habit of visiting the house of the high priest? It is more probable that he was not one of the twelve, but that he is called a disciple, because he had embraced the doctrine of the Son of God.

John is not very exact in arranging the narrative, being satisfied with drawing up a brief summary; for, after having related that Peter once denied Christ, he intermingles other matters, and afterwards returns to the other two denials. Inattentive readers were led by this circumstance to conclude that the first denial took place in the house of Annas. The words, however, convey no such meaning, but rather state clearly that it was the high priest’s maid that constrained Peter to deny Christ. We must, therefore, understand that, when Christ was brought before the high priest, admission was not granted to any person who chose, but that the disciple who was known to the high priest requested, as a personal favor, that Peter might be admitted. There is no reason to doubt that godly zeal was the motive that induced both of them to follow Christ; but since Christ had plainly declared that he spared Peter and the others, he who was so weak would have found it to be far better for him to groan and pray in some dark corner than to go into the presence of men. He now undertakes, with great earnestness, the performance of a duty from which Christ had released him; and when he comes to the confession of faith, in which he ought to have persevered even to death, his courage fails. We ought always to consider what the Lord requires from us, that those who are weak may not undertake what is not necessary.