Charles Spurgeon Commentary Matthew 26:71-72

"And when he was gone out into the porch, another [maid] saw him, and saith unto them that were there, This man also was with Jesus of Nazareth. And again he denied with an oath, I know not the man." — Matthew 26:71-72 (ASV)

There were so many who had seen Peter with Christ that he was easily recognized as one of the Nazarene's companions. His second denial differed from the first, in that he added an oath to the lie, and declared concerning Christ, "I do not know the man." Perhaps the oath was meant to prove that he was no follower of Him who said, "Swear not at all," or it may have been a return to Peter’s old habit before his conversion. Once a child of God gets on the downward road, no one can tell how fast and how far he will fall unless almighty grace is granted to him.