A.T. Robertson Commentary John 6:64

"But there are some of you that believe not. For Jesus knew from the beginning who they were that believed not, and who it was that should betray him." — John 6:64 (ASV)

That believe not (ο ου πιστευουσιν). Failure to believe kills the life in the words of Jesus.

Knew from the beginning (ηιδε εξ αρχης). In the N.T. we have εξ αρχης only here and 16:4, but απ' αρχης in apparently the same sense as here in 15:27; 1 John 2:7,24; 3:11 and see Lu 1:2; 1 John 1:1. From the first Jesus distinguished between real trust in him and mere lip service (2:24; 8:31), two senses of πιστευω.

Were (εισιν). Present active indicative retained in indirect discourse.

And who it was that should betray him (κα τις εστιν ο παραδωσων). Same use of εστιν and note article and future active participle of παραδιδωμ, to hand over, to betray. John does not say here that Jesus knew that Judas would betray him when he chose him as one of the twelve, least of all that he chose him for that purpose. What he does say is that Jesus was not taken by surprise and soon saw signs of treason in Judas. The same verb is used of John's arrest in Mt 4:12. Once Judas is termed traitor (προδοτης) in Lu 6:16. Judas had gifts and was given his opportunity. He did not have to betray Jesus.