John 6:64

But there are some of you that believe not - This is addressed to Judas, and to those disciples who left him: John 6:66.

And who should betray him - Or, who would deliver him up. Because he knew all things; he knew from the first, from Judas's call to the apostleship, and from eternity, (if the reader pleases), who it was who would (not should) deliver him up into the hands of the Jews. Should, in the apprehension of most, implies necessity and compulsion; would implies that he was under the influence of his own free will, without necessity or constraint. The former takes away his guilt: for what a man is irresistibly compelled to do, by the supreme authority of God, he cannot avoid; and therefore to him no blame can attach: but Judas having acted through his own free will, abusing his power, and the grace he had received, he was guilty of the murder of an innocent man, and deserved the perdition to which he went.

Jesus knew from the beginning ... - As this implied a knowledge of the heart, and of the secret principles and motives of men, it shows that he must have been omniscient.

6:64 But there are some of you who believe not - And so receive no life by them, because you take them in a gross literal sense.For Jesus knew from the beginning - Of his ministry: who would betray him - Therefore it is plain, God does foresee future contingencies: "But his foreknowledge causes not the fault, Which had no less proved certain unforeknown."