John 6:65

But there are some, &c. The reason why some of you do not receive, but oppose, My words concerning the Eucharist, is not because My saying is hard, as ye say, but because ye are faithless, and will not believe My many miracles and signs. For here there is need of humble faith, which ought by lowly prayer to be asked and waited for from God the Father. But ye lack humility both of prayer and faith, and therefore ye neither pray to God, nor believe in Me. So S. Augustine, Bede and Rupert.

For Jesus knew, &c. It means that Christ as God knew from eternity what would happen, and this foreknowledge He communicated to His Humanity from the beginning of His conception. And who should betray Him. By this John intimates that Judas the traitor was one of those who did not believe; indeed, that he was offended at Christ's sayings concerning the eating His flesh: that he conceived and cherished a dislike to Christ which at last broke out into treachery against Him. The connection makes this conclusion necessary. Otherwise this mention of the traitor would be inopportune, unless from this discourse of Christ Judas had taken the first initiative of his unbelief and subsequent treachery. So S. Augustine, Bede, &c

Christ added this that the Jews might not think that He had, unaware of his future treachery, admitted Judas to the Apostolate. He had done it consciously and advisedly, that so His Passion and man's redemption might be fulfilled as God had decreed.