9 For when he says, “The flesh lusts against the Spirit, and the Spirit against the flesh;” do not suppose that so much has been attributed to the spirit of man. It is the Spirit of God who fights in you against yourself, against that which in you is against you. For you would not stand to Godward; you fell, wast broken; as a vessel when it falls from a man's hand to the ground, were you broken. And because you were broken, therefore are you turned against yourself; therefore are you contrary to your own self.
Let there be nought in you contrary to yourself, and you shall stand in your integrity. For that you may know that this office appertains to the Holy Spirit; the Apostle says in another place, “For if you live after the flesh, you shall die; but if you through the Spirit do mortify the deeds of the flesh, you shall live.” From these words man was at once uplifting himself, as though by his own spirit he were able to mortify the deeds of the flesh. “If you live after the flesh, you shall die; but if through the Spirit ye do mortify the deeds of the flesh, you shall live.”
Explain to us, Apostle, through what spirit? For man also has a spirit appertaining to his proper nature, whereby he is man. For man consists of body and spirit. And of this spirit of man it is said, “No man knows the things of a man, save the spirit of man which is in him.” I see then that man himself has his own spirit appertaining to his proper nature, and I hear you saying, “But if through the Spirit ye do mortify the deeds of the flesh, you shall live.” I ask, through what spirit; my own, or God's?
For I hear your words, and am still perplexed by this ambiguity. For when the word “spirit” is used, it is used sometimes of the spirit of a man, and of cattle, as it is written, that “all flesh which had in itself the spirit of life, died by the flood.” And so the word spirit is spoken of cattle, and spoken of man too. Sometimes even the wind is called spirit; as it is in the Psalm, “Fire, hail, snow, frost, the spirit of the tempest.” For as much then as the word “spirit” is used in many ways, by what spirit, O Apostle, have you said that the deeds of the flesh are to be mortified; by my own, or by the Spirit of God?
Hear what follows, and understand. The difficulty is removed by the following words. For when he had said, “But if through the Spirit ye mortify the deeds of the flesh, you shall live;” he added immediately, “For as many as are acted upon by the Spirit of God, they are the sons of God.” Thou dost act, if you are acted upon, and actest well, if you are acted upon by the Good. So then when he said to you, “If through the Spirit ye mortify the deeds of the flesh, you shall live;” and it was doubtful with you of what spirit he had spoken, in the words following understand the Master, acknowledge the Redeemer.
For That Redeemer has given you the Spirit Whereby you may mortify the deeds of the flesh. “For as many as are acted upon by the Spirit of God, they are the sons of God.” They are not the sons of God if they are not acted upon by the Spirit of God. But if they are acted upon by the Spirit of God, they fight; because they have a mighty Helper. For God does not look on at our combattings as the people do at the gladiators. The people may favour the gladiator, help him they cannot when he is in peril.
Source: Sermons on Selected Lessons of the New Testament (New Advent)