John 6:60-65

Many therefore of his disciples, when they heard this, said, This is a hard saying; who can hear it? But Jesus knowing in himself that his disciples murmured at this, said unto them, Doth this cause you to stumble? What then if ye should behold the Son of man ascending where he was before? It is the spirit that giveth life; the flesh profiteth nothing: the words that I have spoken unto you are spirit, and are life. But there are some of you that believe not. For Jesus knew from the beginning who they were that believed not, and who it was that should betray him. And he said, For this cause have I said unto you, that no man can come unto me, except it be given unto him of the Father.