What does John 13:38 mean?

Peter has a habit of speaking without understanding. Earlier in the evening, he resisted Jesus' washing of the disciples' feet (John 13:6–10). In the prior verse, he declared he was willing and able to go anywhere with Jesus, even to death (John 13:37). That was in response to Jesus saying that where He was going others would not be able to follow (John 13:36). On top of this, Jesus has predicted that one of the inner circle is a traitor (John 13:21). No one yet realizes this is Judas, who has already left to alert the authorities (John 13:26–30). What Peter says is both a boast, and a declaration that he is certainly not the betrayer.

That makes Jesus' response here both confusing and devastating to Peter, as well as the other disciples. Jesus uses the famous "truly, truly," phrasing here. This repeats the word amēn, which carried through Aramaic into Greek. This phrase means what Jesus is about to say is absolute, personal, incontrovertible truth. The truth is that despite Peter's bravado, he is hours away from entirely denying that he has anything to do with Jesus (John 18:27). That moment will shatter Peter's self-confidence (Matthew 26:71–75), but he will eventually be restored (Mark 16:7).

To the other disciples—and possibly even to Peter—this comment would raise suspicion that Peter is the betrayer spoken of earlier. He is not, and Judas will soon be discovered. But for the moment, it's likely that the other disciples looked at Peter with distrust.