5 Therefore mark, my brethren, what answer this man who came to Jesus by night makes. Although he came to Jesus, yet because he came by night, he still speaks from the darkness of his own flesh. He understands not what he hears from the Lord, understands not what he hears from the Light, “which lights every man that comes into this world.” Already has the Lord said to him, “Unless a man be born again, he shall not see the kingdom of God. Nicodemus says unto Him, How can a man be born again when he is old?”
The Spirit speaks to him, and he thinks of the flesh. He thinks of his own flesh, because as yet he thinks not of Christ's flesh. For when the Lord Jesus had said, “Except a man eat my flesh, and drink my blood, he shall not have life in him,” some who followed Him were offended, and said among themselves, “This is a hard saying; who can hear it?” For they fancied that, in saying this, Jesus meant that they would be able to cook Him, after being cut up like a lamb, and eat Him: horrified at His words, they went back, and no more followed Him.
Thus speaks the evangelist: “And the Lord Himself remained with the twelve; and they said to Him, Lo, those have left You. And He said, Will ye also go away?”— wishing to show them that He was necessary to them, not they necessary to Christ. Let no man fancy that he frightens Christ, when he tells Him that he is a Christian; as if Christ will be more blessed if you be a Christian. It is a good thing for you to be a Christian; but if you be not, it will not be ill for Christ. Hear the voice of the psalm, “I said to the Lord, You are my God, since You have no need of my goods.” For that reason, “You are my God, since of my goods You have no need.”
If you be without God, you will be less; if you be with God, God will not be greater. Not from you will He be greater, but you without Him will be less. Grow, therefore, in Him; do not withdraw yourself, that He may, as it were, diminish. You will be renewed if you come to Him, wilt suffer loss if you depart from Him. He remains entire when you come to Him, remains entire even when you fall away. When, therefore, He had said to His disciples, “Will ye also go away?” Peter, that Rock, answered with the voice of all, “Lord, to whom shall we go?
You have the words of eternal life.” Pleasantly savored the Lord's flesh in his mouth. The Lord, however, expounded to them, and said, “It is the Spirit that quickens.” After He had said, “Except a man eat my flesh, and drink my blood, he shall not have life in him,” lest they should understand it carnally, He said, “It is the Spirit that quickens, but the flesh profits nothing: the words which I have spoken unto you are spirit and life.”
Source: Tractates on the Gospel of John (New Advent)