I may say what I will to you; the Spirit quickeneth, but it doth not quicken all; it only quickeneth whomsoever it pleaseth. You understand not these things, but have most gross conceptions of sublime spiritual things; the reason is, because you believe not: though some of them, questionless, did truly believe, yet the most did not; for we read, 1 Thessalonians 6:66, that many of them went back, and walked no more with him. And though faith be an inward, secret act of the soul, yet Christ knew, and from the beginning, who were believers, and who were not; nay, he had a particular knowledge of that disciple who was to betray him.
Related Commentaries of John 6:64
John 6:64
64 But there are some of you that believe not. For Jesus knew from the beginning who they were that believed not, and who should betray him.