6 Hence “the words,” says He, “which I have spoken to you are Spirit and life.” For we have said, brethren, that this is what the Lord had taught us by the eating of His flesh and drinking of His blood, that we should abide in Him and He in us. But we abide in Him when we are His members, and He abides in us when we are His temple. But that we may be His members, unity joins us together. And what but love can effect that unity should join us together? And the love of God, whence is it? Ask the apostle: “The love of God,” says he, “is shed abroad in our hearts by the Holy Spirit which is given to us.” Therefore “it is the Spirit that quickens,” for it is the Spirit that makes living members. Nor does the Spirit make any members to be living except such as it finds in the body, which also the Spirit itself quickens. For the Spirit which is in you, O man, by which it consists that you are a man, does it quicken a member which it finds separated from your flesh? I call your soul your spirit. Your soul quickens only the members which are in your flesh; if you take one away, it is no longer quickened by your soul, because it is not joined to the unity of your body. These things are said to make us love unity and fear separation. For there is nothing that a Christian ought to dread so much as to be separated from Christ's body. For if he is separated from Christ's body, he is not a member of Christ; if he is not a member of Christ, he is not quickened by the Spirit of Christ. “But if any man,” says the apostle, “have not the Spirit of Christ, he is none of His.” “It is the Spirit,” then, “that quickens; the flesh profits nothing. The words that I have spoken to you are spirit and life.” What means “are spirit and life”? They are to be understood spiritually. Have you understood spiritually? “They are spirit and life.” Have you understood carnally? So also “are they spirit and life,” but are not so to you.
7. “But,” says He, “there are some among you that believe not.” He said not, There are some among you that understand not; but He told the cause why they understand not. “There are some among you that believe not,” and therefore they understand not, because they believe not. For the prophet has said, “If you believe not, you shall not understand.” We are united by faith, quickened by understanding. Let us first adhere to Him through faith, that there may be that which may be quickened by understanding. For he who adheres not resists; he that resists believes not. And how can he that resists be quickened? He is an adversary to the ray of light by which he should be penetrated: he turns not away his eye, but shuts his mind. “There are,” then, “some who believe not.” Let them believe and open, let them open and be illumined. “For Jesus knew from the beginning who they were that believed, and who should betray Him.” For Judas also was there. Some indeed, were offended; but he remained to watch his opportunity, not to understand. And because he remained for that purpose, the Lord kept not silence concerning him. He described him not by name, but neither was He silent about him; that all might fear though only one should perish. But after He spoke, and distinguished those that believe from those that believe not, He clearly showed the cause why they believed not. “Therefore I said unto you,” says He, “that no man can come unto me except it were given to him of my Father.” Hence to believe is also given to us; for certainly to believe is something. And if it is something great, rejoice that you have believed, yet be not lifted up; for “What do you have that you did not receive?”
8. “From that time many of His disciples went back, and walked no more with Him.” Went back, but after Satan, not after Christ. For our Lord Christ once addressed Peter as Satan, rather because he wished to precede his Lord, and to give counsel that He should not die, He who had come to die, that we might not die for ever; and He says to him, “Get behind me, Satan; for you savor not the things that be of God, but the things that be of men.” He did not drive him back to go after Satan, and so called him Satan; but He made him go behind Himself, that by walking after his Lord he should not be a Satan. But these went back in the same manner as the apostle says of certain women: “For some are turned back after Satan.” They walked not further with Him. Behold, cut off from the body, for perhaps they were not in the body, they have lost life. They must be reckoned among the unbelieving, notwithstanding they were called disciples. Not a few, but “many went back.” This happened, it may be, for our consolation. For sometimes it happens that a man may declare the truth, and that what he says may not be understood, and so they that hear it are offended and go away. Now the man regrets that he had spoken that truth, and he says to himself, “I ought not to have spoken so, I ought not to have said this.” Behold; it happened to the Lord: He spoke, and lost many; He remained with few. But yet He was not troubled, because He knew from the beginning who they were that believed and that believed not. If it happen to us, we are sorely perplexed. Let us find comfort in the Lord, and yet let us speak words with prudence.
Source: Tractates on the Gospel of John (New Advent)