Chapter 6:66-71 Peter's Confession of Faith

6:66 From that time many of His disciples went back, and walked no more with Him.
Many of His disciples went back - They no longer associated with Him, nor professed to acknowledge Him as the Messiah. None of these were of the twelve.

6:67 Then said Jesus unto the twelve, Will ye also go away?
Will ye also go away? - These words are very emphatical. Will You abandon Me?

6:68 Then Simon Peter answered Him, Lord, to whom shall we go? Thou hast the words of eternal life.

That was a very conclusive way of answering one question by another: “Will ye also go away?” “Lord, to whom shall we go? Thou hast the words of eternal life.”

6:69 And we believe and are sure that Thou art that Christ, the Son of the living God.

the son of the living God - this they knew, and were sure of, both by John's testimony, and by the Father's voice from heaven; which three of them heard, at Christ's transfiguration on the mount.

6:70 Jesus answered them, Have not I chosen you twelve, and one of you is a devil?

Hypocrites and betrayers of Christ are no better than devils. 

Judas not only had a devil, 

but he was a devil.

6:71 He spake of Judas Iscariot the son of Simon: for he it was that should betray Him, being one of the twelve.

When a man forsakes the living God, and gives way to avarice, which appears to have been the case with Judas, he is fit for any thing in which Satan may choose to employ him. 

Beware of the love of money! The cursed lust of gold induced a disciple of Christ to betray his God: and has it not been the ruin of millions since?