What does John 13:28 mean?

John has described a scene with many interacting elements. Jesus and the disciples are reclining to eat a meal together, during which Jesus makes an emotional prediction that one of these men will betray Him (John 13:21). That inspired confusion and various efforts to find out which one was the culprit (John 13:23–25; Matthew 26:22–23). Amid that hubbub, Jesus explains a specific sign, indicating Judas (John 13:26). He then tells the Satan-possessed man to hurry to accomplish his treachery (John 13:27).

Based on the reactions of the disciples—including this verse—it's clear none of the other men put these pieces together until after the fact. The following verse proves that when Judas leaves, it's assumed he's on an errand (John 13:29). This is especially important to remember when reading about Jesus predicting Peter's denial (John 13:38). In that moment, it's easy to assume that most of the disciples thought Peter, not Judas, was the most likely suspect.

Matthew provides additional details about the conversation between Jesus and Judas. Those, like the information given by John, are fragments of a complete picture none of these men saw at the time (Matthew 26:23–25).