2 “But Jesus was followed,” he says, “by Simon Peter, and another disciple.” Who that other disciple is, we cannot affirm with confidence, because it is left unnoticed here. But it is in this way that John usually refers to himself, with the addition, “whom Jesus loved.” Perhaps, therefore, it is he also in the present case; but whoever it is, let us look at what follows. “And that disciple,” he says, “was known unto the high priest, and went in with Jesus into the palace of the high priest; but Peter stood at the door without.
Then went out that other disciple, who was known unto the high priest, and spoke unto her that kept the door, and brought in Peter. Then says the damsel that kept the door unto Peter, Are you also one of this man's disciples? He says, I am not.” Lo, the pillar of greatest strength has at a single breath of air trembled to its foundations. Where is now all that boldness of the promiser, and his overweening confidence in himself beforehand? What now of those words, when he said, “Why cannot I follow You now?
I will lay down my life for Your sake.” Is this the way to follow the Master, to deny his own discipleship? Is it thus that one's life is laid down for the Lord, when one is frightened at a maid-servant's voice, lest it should compel us to the sacrifice? But what wonder, if God foretold what was true, and man presumptuously imagined what was false? Assuredly in this denial of the Apostle Peter, which had now entered on its first stage, we ought to take notice that not only is Christ denied by one who says that He is not Christ, but by him also who, while really a Christian, himself denies that he is so.
For the Lord said not to Peter, You shall deny that you are my disciple; but, “You shall deny me.” Him, therefore, he denied, when he denied that he was His disciple. And what else did such a form of denial imply, but that of his own Christianity? For although the disciples of Christ were not yet called by such a name—because it was after His ascension, in Antioch, first that the disciples began to be called Christians, — yet the thing itself, that afterwards assumed such a name, already existed, those who were afterwards called Christians were already disciples; and this common name, like the common faith, they transmitted to their posterity.
He, therefore, who denied that he was Christ's disciple, denied the reality of the thing, of which the being called a Christian was only the name. How many afterwards, not to speak of old men and women, whose satiated feelings as regards the present life might more easily enable them to brave death for the confession of Christ; and not merely the youth of both sexes, when of an age at which the exercise of fortitude seems to be fairly required; but even boys and girls could do— even as an innumerable company of holy martyrs with brave hearts and by a violent death entered the kingdom of heaven— what at that moment he was unable to do, who received the keys of that kingdom. It is here we see why it was said, “Let these go their way,” when He, who has redeemed us by His own blood, gave Himself for us; that the saying which He spoke might be fulfilled, “Of those whom You have given me I have lost none.”
For assuredly, had Peter gone hence after denying the Christ, what else would have awaited him but destruction?
Source: Tractates on the Gospel of John (New Advent)