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.
Source: Tractates on the Gospel of John (New Advent)