24 It may be, however, that some one will say to us: Peter had not actually gone out as yet, but had only risen with the purpose of going out. This may be the allegation of one who is of opinion that the second interrogation and denial took place when Peter was outside at the door. Let us therefore look at what follows in John's narrative. It is to this effect: “The high priest then asked Jesus of His disciples, and of His doctrine. Jesus answered him, I spoke openly to the world; I ever taught in the synagogue, and in the temple, whither the Jews always resort; and in secret have I said nothing.
Why do you ask me? Ask them which heard me what I have said unto them: behold, they know what I said. And when He had thus spoken, one of the officers which stood by struck Jesus with the palm of his hand, saying, Do you answer the high priest so? Jesus answered him, If I have spoken evil, bear witness of the evil; but if well, why do you smite me? And Annas sent Him bound to Caiaphas the high priest.” This certainly shows us that Annas was high priest. For Jesus had not been sent to Caiaphas as yet, when the question was thus put to Him, “Do you answer the high priest so?”
Mention is also made of Annas and Caiaphas as high priests by Luke at the beginning of his Gospel. After these statements, John reverts to the account which he had previously begun of Peter's denial. Thus he brings us back to the house in which the incidents took place which he has recorded, and from which Jesus was sent away to Caiaphas, to whom He was being conducted at the commencement of this scene, as Matthew has informed us. Moreover, it is in the way of a recapitulation that John records the matters regarding Peter which he has introduced at this point.
Falling back upon his narration of that incident with the view of making up a complete account of the threefold denial, he proceeds thus: “And Simon stood and warmed himself. They said therefore unto him, Are you not also one of his disciples? He denied it, and said, I am not.” Here, therefore, we find that Peter's second denial occurred, not when he was at the door, but as he was standing by the fire. This, however, could not have been the case, had he not returned by this time after having gone outside.
For it is not that by this second occasion he had actually gone out, and that the other maid who is referred to saw him there outside; but the matter is put as if it was on his going out that she saw him; or, in other words, it was when he rose to go out that she observed him, and said to those who were there—that is, to those who were gathered by the fire inside, within the court—“This fellow was also with Jesus of Nazareth.” Then we are to suppose that the man who had thus gone outside, on hearing this assertion, came in again, and swore to those who were now inimically disposed, “I do not know the man.” In like manner, Mark also says of this same maid, that “she began to say to them that stood by, This is one of them.” For this damsel was speaking not to Peter, but to those who had remained there when he went out.
At the same time, she spoke in such a manner that he heard her words; whereupon he came back and stood again by the fire, and met their words with a negative. Then we have the statement made by John in these terms: “They said, Are you not also one of his disciples?” We understand this question to have been addressed to him on his return as he stood there; and we also recognise the harmony in which this stands with the position that on this occasion Peter had to do not only with that other maid who is mentioned by Matthew and Mark in connection with this second denial, but also with that other person who is introduced by Luke.
This is the reason why John uses the plural, “They said.” The explanation then may be, that when the maid said to those who were with her in the court as he went out, “This is one of them,” he heard her words and returned with the purpose of clearing himself, as it were, by a denial. Or, in accordance with the more probable theory, we may suppose that he did not catch what was said about him as he went out, and that on his return the maid and the other person who is introduced by Luke addressed him thus, “Are you not also one of his disciples?” that he met them with a denial, “and said, I am not;” and further, that when this other person of whom Luke speaks insisted more pertinaciously, and said, “Surely you are one of them,” Peter answered thus, “Man, I am not.”
Still, when we compare together all the statements made by the several evangelists on this subject, we come clearly to the conclusion, that Peter's second denial took place, not when he was at the door, but when he was within, by the fire in the court. It becomes evident, therefore, that Matthew and Mark, who have told us how he went without, have left the fact of his return unnoticed simply with a view to brevity.
Source: The Harmony of the Gospels (New Advent)