“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.”
John 6:64 — Greek Interlinear
Church Fathers on John 6:64
Such is our Lord’s discourse. The people did not perceive that it had a deep meaning or, that grace went along with it: but receiving the matter in their own way, and taking His words in a human sense, understood Him as if He spoke of cutting of the flesh of the Word into pieces, for distribution to those who believed on Him: Many therefore, not of His enemies, but even of His disciples, when they heard this, said, This is a hard saying, who can bear it?
i.e. difficult to receive, too much for their weakness. They thought He spoke above Himself, and more loftily than He had a right to do; and so said they, Who can bear it? which was answering in fact for themselves, that they could not.
When you hear, however, of His disciples murmuring, understand not those really such, but rather some who, as far as their air and behavior went, seemed to be receiving instruction from Him. For among His disciples were some of the people, who were called such, because they stayed some time with His disciples.