What does John 6:71 mean?
Judas's betrayal did not take Jesus by surprise. Even in this story, Jesus has demonstrated that He knows what other people are really thinking (John 6:26). Judas stayed with Jesus even when many others left (John 6:66). Peter, in response, claims that those who have remained genuinely believe that Jesus is "the Holy One of God" (John 6:68–69). Judas, obviously, does not, and Jesus knows this.His reasons for telling the group that one of them is an unbeliever are unclear, at least at this point. Jesus earlier separated these twelve men from the crowd, right after the people clamored for Jesus to become king. It's possible that this was done to keep the disciples humble. Jesus' disturbing remark that one of these men, even now, was "a devil" might have been intended to have the same effect. Rather than being overly proud that they'd stayed with Jesus, the disciples needed to be reminded that, even so, things are not always as good as they seem.