6 What struggle? “But if you bite and devour one another, take heed that you be not consumed one of another. But I say, Walk in the Spirit.” I am quoting the words of the Apostle, which have been just read out of his Epistle. “But I say, Walk in the Spirit, and you shall not fulfil the lusts of the flesh.” “But I say, Walk in the Spirit, and the lusts of the flesh,” he did not say, “You shall not have;” nor did he say, “You shall not do;” but, “You shall not fulfil.”
Now what this is, with the Lord's assistance, I will declare as I shall be able; give attention, that you may understand, if you are walking in the Spirit. “But I say, Walk in the Spirit, and you shall not fulfil the lusts of the flesh.” Let him follow on; if haply anything, as this which is here obscure, may be understood more easily by the sequel of his words. For I said, that it was not without a meaning that the Apostle would not say, “You shall not have the lusts of the flesh;” nor again would even say, You shall not do the lusts of the flesh; but said, “You shall not fulfil the lusts of the flesh.”
He has set forth this struggle before us. In this battle are we occupied, if we are in God's service. What then follows? “For the flesh lusts against the spirit, and the spirit against the flesh. For these are contrary the one to the other, so that you do not the things that you would.” This, if it be not understood, is with exceeding peril heard. And therefore anxious as I am lest men by an evil interpretation should perish, I have undertaken with the Lord's assistance to explain these words to your affection. We have leisure enough, we have begun early in the morning, the hour of dinner does not press; on this day, the sabbath that is, they that hunger after the word of God are wont especially to meet together. Hear and attend, I will speak with what carefulness I can.
Source: Sermons on Selected Lessons of the New Testament (New Advent)