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.
John 6:60–71 shows the drastic impact of Jesus' teaching on the crowd: most walk away. When confronted with spiritual needs and a spiritual message, most people will turn it down. What society wants is spectacle, material things, and a political savior. The more Jesus insists on being the means to eternal life, the angrier the crowd becomes, until the vast majority simply abandon Him. The twelve disciples, on the other hand, seem willing to follow Jesus, though they are also struggling to accept His recent claims. Peter's declaration will be the third of John's seven witnesses to Jesus' divinity.
In chapter 6, Jesus feeds thousands of people who had been following Him. He does this by miraculously dividing the contents of a small meal, leaving more left over than He had to begin with. At first, the crowd is amazed and they enthusiastically praise Jesus. After sending the disciples across the Sea of Galilee and rescuing them from a storm by walking on the water, Jesus once again addresses the crowd. This time, He emphasizes the spiritual lesson behind His prior miracle. In response, most of those who had been praising Jesus turn away in disappointment.