But he denied before them all, saying, I know not what thou sayest.

People's New Testament

26:70 But he denied before them all. Denied that he was with Jesus of Galilee (Mt 26:69). But a few hours before Peter had asserted that though all others deserted the Lord he would not, and that he would die with him, and when Judas led the band into Gethsemane, Peter, refusing to consider the odds, flung himself upon them, valiant as a lion, struck and wounded Malchus, and would probably have slain him had he not swerved. He was a brave as a hero then--now is timid as a deer. The explanation is that his faith had failed when he saw his Master apparently helpless in the hands of his enemies. See Heb 11:32-35.

Matthew Henry's Concise Commentary

26:69-75 Peter's sin is truly related, for the Scriptures deal faithfully. Bad company leads to sin: those who needlessly thrust themselves into it, may expect to be tempted and insnared, as Peter. They scarcely can come out of such company without guilt or grief, or both. It is a great fault to be shy of Christ; and to dissemble our knowledge of him, when we are called to own him, is, in effect, to deny him. Peter's sin was aggravated; but he fell into the sin by surprise, not as Judas, with design. But conscience should be to us as the crowing of the cock, to put us in mind of the sins we had forgotten. Peter was thus left to fall, to abate his self-confidence, and render him more modest, humble, compassionate, and useful to others. The event has taught believers many things ever since, and if infidels, Pharisees, and hypocrites stumble at it or abuse it, it is at their peril. Little do we know how we should act in very difficult situations, if we were left to ourselves. Let him, therefore, that thinketh he standeth, take heed lest he fall; let us all distrust our own hearts, and rely wholly on the Lord. Peter wept bitterly. Sorrow for sin must not be slight, but great and deep. Peter, who wept so bitterly for denying Christ, never denied him again, but confessed him often in the face of danger. True repentance for any sin will be shown by the contrary grace and duty; that is a sign of our sorrowing not only bitterly, but sincerely.