John 6:71

Now he spake of Judas the son of Simon Iscariot, for he it was that should betray, him, being one of the twelve.

Perhaps the purpose of Jesus' introduction of this shocking revelation was to prepare the other apostles for the impact of so dastardly a deed as the betrayal; and there could have been no better time for such a warning than the very moment when Peter was affirming so strongly their faith and knowledge of the Son of God. This is another example of the sense of movement throughout this Gospel, a characteristic which some, incredibly, have failed to see.

The placement of this warning concerning Judas at this particular place in the Gospel supports the supposition that Judas had been taken in by the arguments of those who wanted to make Jesus king, and that the traitor found the demands of his carnal nature more in harmony with the enemies of the Lord than in companionship with the Lord of life.