5 What is it, then, that He adds? “It is the Spirit that quickens; the flesh profits nothing.” Let us say to Him (for He permits us, not contradicting Him, but desiring to know), O Lord, good Master, in what way does the flesh profit nothing, while You have said, “Except a man eat my flesh, and drink my blood, he shall not have life in him?” Or does life profit nothing? And why are we what we are, but that we may have eternal life, which Thou dost promise by Your flesh?
Then what means “the flesh profits nothing”? It profits nothing, but only in the manner in which they understood it. They indeed understood the flesh, just as when cut to pieces in a carcass, or sold in the shambles; not as when it is quickened by the Spirit. Wherefore it is said that “the flesh profits nothing,” in the same manner as it is said that “knowledge puffs up.” Then, ought we at once to hate knowledge? Far from it! And what means “Knowledge puffs up”? Knowledge alone, without charity.
Therefore he added, “but charity edifies.” Therefore add to knowledge charity, and knowledge will be profitable, not by itself, but through charity. So also here, “the flesh profits nothing,” only when alone. Let the Spirit be added to the flesh, as charity is added to knowledge, and it profits very much. For if the flesh profited nothing, the Word would not be made flesh to dwell among us. If through the flesh Christ has greatly profited us, does the flesh profit nothing? But it is by the flesh that the Spirit has done somewhat for our salvation.
Flesh was a vessel; consider what it held, not what it was. The apostles were sent forth; did their flesh profit us nothing? If the apostles' flesh profited us, could it be that the Lord's flesh should have profited us nothing? For how should the sound of the Word come to us except by the voice of the flesh? Whence should writing come to us? All these are operations of the flesh, but only when the spirit moves it, as if it were its organ. Therefore “it is the Spirit that quickens; the flesh profits nothing,” as they understood the flesh, but not so do I give my flesh to be eaten.
Source: Tractates on the Gospel of John (New Advent)