Albert Barnes on Matthew 26:70

But he denied before them all, etc. He denied that he was a disciple; he denied that he knew Jesus; he denied (Mark) that he understood what was meant---i. e., he did not see any reason why this question was asked. All this was palpable falsehood; and Peter must have known that it was such. This is remarkable, because Peter had just before been so confident. It is more remarkable, because the edge of the charge was taken off by the insinuation that John was known to be a disciple--thou also wast with Jesus of Galilee.