John Calvin on Matthew 26:74

74. Then he began to curse and to swear. In this third denial, Peter’s unfaithfulness to his Master reached its utmost height. Not satisfied with swearing, he breaks out into cursing, by which he abandons his body and soul to destruction; for he prays that the curse of God may fall upon him, if he knows Christ. It is as much as if he had said, May I perish miserably, if I have any thing in common with the salvation of God! So much the more ought we to admire the goodness of Christ, who rescued his disciple from such fatal ruin, and healed him. Now this passage shows, that when a man falling through weakness of the flesh, denies the truth though he knows it, this does not amount to “blasphemy against the Holy Spirit” (Matthew 12:31, 32.) Peter had unquestionably heard from the mouth of the Lord how detestable treachery it is to deny him before men; and what dreadful vengeance, before God and before his angels, (Matthew 10:39 Luke 12:9) awaits those who, through a cowardly dread of the cross, abandon the confession of faith; and not without reason had he, a little before, preferred death and every kind of torment to denying Christ. Now, therefore, he throws himself down knowingly, and after previous warning; but afterwards he obtains pardon; from which it follows that he sinned through weakness and not through incurable malice. For he would willingly have rendered to Christ the duties of friendship which he owed him, had not fear extinguished the sparks of proper affection.