John Calvin on John 6:64

64. But there are some of you who do not believe. He again imputes blame to them, because, being destitute of the Spirit, they wickedly corrupt and debase his doctrine, and thus turn it to their ruin. For otherwise they might have objected: “You boast, indeed, that what you speak is quickening, but we experience nothing of that nature.” He therefore says, that by themselves it is prevented; for unbelief, as it is always proud, will never understand any thing in the words of Christ which it despises and disdains. Wherefore, if we wish to profit at all under this Teacher, let us bring minds well disposed to listen to him; for if the entrance to his doctrine be not opened up by humility and reverence, our understandings are harder than stones, and will not receive any part of sound doctrine. And therefore, when in the present day we see so few people in the world profiting by the Gospel, we ought to remember that this arises from the depravity of men. For how many will you find who deny themselves, and truly submit to Christ? As to his saying only that there were some who did not believe, though almost all of them were liable to this charge, his reason for doing so appears to have been that, if there were any who were not yet beyond the possibility of cure, they might not cast down their minds in despair.

For Jesus knew from the beginning. The Evangelist added this, that none might think that Christ formed an opinion at random about his hearers. Many professed to belong to his flock, but a sudden apostacy exposed their hypocrisy. But the Evangelist says that their treachery, even while it was unknown to others, was well known to Christ. And this is stated, not so much on his account, as that we may learn not to form a judgment except on subjects which we have thoroughly investigated; for as to their being known to Christ from the beginning, this was peculiar to his Divinity. It is otherwise with us; for since we do not know the hearts, we ought to delay forming a judgment, until impiety be manifested by outward signs, and thus the tree be known by its fruits, (Matthew 7:16.)

John 6:65-71

65. And he said, Therefore have I told you that no man can come to me, unless it be given to him by my Father. 66. From that time many of his disciples went back, and walked no more with him. 67. Jesus therefore said to the twelve, Do you also wish to go away? 68. Simon Peter therefore answered him, Lord, to whom shall we go? Thou hast the words of eternal life. 69. And we have believed and known that thou art the Christ, the Son of the living God. 70. Jesus answered them, Have not I chosen you twelve, and one of you is a devil? 71. Now he spoke of Judas Iscariot, son of Simon; for it was he that would betray him, although he was one of the twelve.