Mark 14:69

(ubi sup.) By this denial of Peter we learn, that not only he denies Christ, who says that He is not the Christ, but he also, who although he is a Christian, denies himself to be such. For the Lord did not say to Peter, Thou shalt deny thyself to be my disciple, but, Thou shalt deny me; he therefore denied Christ, when he said that he was not His disciple.

Again, when the maid saw him, she began to say to those standing around: This man is one of them. But he denied it again. The one accusing him now should not be believed to be the same as before. For Matthew says very clearly: When he went out to the gate, another saw him and said to those who were there (Matthew 26), etc. From this denial of Peter, we learn not only that Christ is denied by someone who says He is not Christ, but also by someone who, when He is, denies that he himself is a Christian. However, the Lord did not say to Peter: You will deny my disciple, but: You will deny me. So he denied Him when he denied being His disciple.

(ubi sup.)y This maid is not the same, but another, as Matthew says. Indeed we must also understand, that in this second denial he was addressed by two persons, that is, by the maid whom Matthew and Mark mention, and by another person, of whom Luke takes notice. It goes on: And he denied it again. Peter had now returned, for John says that he denied Him again standing at the fire; wherefore the maid said what has been mentioned above, not to him, that is, Peter, but to those who, when he went out, had remained, in such a way however that he heard it; wherefore coming back and standing again at the fire, he contradicted them, and denied their words. For it is evident, if we compare the accounts of all the Evangelists on this matter, that Peter did not the second time deny him before the porch, but within the palace at the fire, whilst Matthew and Mark who mention his having gone out are silent, for the sake of brevity, as to his return.