Origen

But you will ask, whether it were possible that Peter should not have been offended, when once the Saviour had said, All ye shall be offended in me. To which one will answer, what is foretold by Jesus must of necessity come to pass; and another will say, that He who at the prayer of Ninevites turned away the wrath He had denounced by Jonas, might also have averted Peter’s offence at his entreaty. But his presumptuous confidence, prompted by zeal indeed but not a cautious zeal, became the cause not only of offence but of a thrice repeated denial. And since He confirmed it with the sanction of an oath, some one will say that it was not possible that he should not have denied Him. For Christ would have spoken falsely when he said, Verily I say unto thee, if Peter’s assertion, I will not deny thee, had been true. It seems to me that the other disciples having in view not that which was first said, All ye shall be offended, but that which was said to Peter, Verily I say unto thee, &c. made a like promise with Peter because they were not comprehended in the prophecy of denial. Peter said unto him, Though I should die with thee, yet will I not deny thee. Likewise also said all the disciples. Here again Peter knows not what he says; he could not die with Him who was to die for all mankind, who were all in sin, and had need of some one to die for them, not that they should die for others.