What does Matthew 26:25 mean?

While the other disciples are clueless as to his treachery, Judas knows he is the betrayer Jesus had spoken of (Matthew 26:20–23). The rest of the disciples immediately asked Jesus if they were the guilty one, but He has only said that it would have been better for the betrayer if he had never been born, pronouncing a judgment of "woe" on the man (Matthew 26:24).

Now Judas finally asks what the others have already asked, with a difference. The others had called Jesus "Lord." Scripture does not say, explicitly, that everyone but Judas had asked that exact question. It's possible that Judas did, continuing to pretend that he was not already guilty (Matthew 26:14–17). Here, however, he responds to Jesus' damnation of the traitor by asking, but not acknowledging Jesus as "Master." Rather, he only calls Him a "teacher," which is not wrong at all (Mark 9:5; John 1:49), but strongly contrasts with the other disciples.

This begs the question of why Judas asked, at all? Perhaps he was trying to cover his guilt by joining in with the others. Perhaps he wanted to see if Jesus already knew he was the guilty one. Or, if Judas had already made his defense (Matthew 26:22), this might be a sarcastic or resigned statement of someone who knows he's caught. In either case, Jesus acknowledges that He knows the truth.

John adds details to the story. Jesus gives a morsel of bread to Judas after dipping it in the bowl that He has mentioned. At that moment, Satan enters fully into Judas (John 13:27). Jesus tells him to do what he is going to do quickly. Judas immediately leaves. The other disciples think Jesus has sent him on an errand, not realizing Judas is the betrayer (John 13:28–30). In the rush of these events happening, such details would have been easily missed until the disciples talked about them later in the week.