Luke 22:60

Luke 22:60. And Peter said, Man, I know not what thou sayest.

Another Evangelist tells us that he began to curse and to swear, as if that was the surest proof that he could possibly give that he did not know Jesus; for, when you hear a man swear, you know at once that he is no Christian, you may conclude that safely enough. So Peter thought that, to prove that he was no follower of Christ, he would use such ill language as the ungodly speak.