"But I say" does not mean: in prescribing the law of love to you, I say the same thing as, "walk by the Spirit." Rather it means: to this, or along with this. I say to you, "walk by the Spirit." In order for you to remain in mutual love, walk by the Spirit and do not fulfill the lust of the flesh.
Why is this so? For this reason: the lust of the flesh is the same as the sin living in us, and elsewhere it is also called the mind of the flesh. This mind of the flesh is not subject to the law of God, nor indeed can be (cf. Rom 8:7). But love is the fulfillment of the Law. It follows that these are incompatible opposites.
The lust of the flesh is the denial of the Law, while love is the denial of every lawlessness. Where love acts in power, there is no place for the lust of the flesh. Where the lust of the flesh is, do not seek love. Whoever obeys the suggestions of love will not begin to obey the temptations of the lust of the flesh.
But in order not to obey the temptations of the flesh, one must receive the grace of the Spirit and walk in the Spirit. It is therefore evident that Saint Paul wished to say to the Galatians: after liberation from the Law, you are obliged to walk in love. In order to walk in love, walk by the Spirit and do not fulfill the lust of the flesh.
What does it mean to walk by the Spirit? We have our own spirit, a Godlike power breathed into man from God, whose purpose is to lead man into life in God and keep him in it. The elements of our spirit are the consciousness of the Godhead, with the feeling of complete dependence on Him, the fear of God, the consciousness of the obligation to please God, and confidence in blessed eternity.
By all these elements the spirit directed man away from himself and from every creature toward God as the final goal, turning everything else into a means toward Him. When man fell, he fell away from God, stopped at himself, and made himself the chief goal of life. The spirit lost authority over him, and self-love began to hover in him instead.
From self-love developed pride, self-interest, and love of pleasure, and from these later came the whole army of passions. All of them, in different shades, became the managers of man's life. The spirit fell silent. If it sometimes raised its voice, it was not heard.
The Lord and Savior brought to earth the grace of the All-Holy Spirit, which, together with the preaching of the Gospel, went forth over the face of the earth. The power of the preaching of the Gospel comes from grace. Along with the word, grace enters the heart through hearing and awakens the human spirit.
The preaching says: "Awake, you who sleep" (Eph 5:14). When grace acts on the spirit, the spirit rises. Together with it, all its elements arise and come to life: the fear of God, the obligation to please God, and the expectation of a better life.
Whoever submits to these demands of the spirit, made alive by the Spirit of God, renounces himself, tramples down self-love with the whole army of passions, and begins to labor for the Lord with all zeal, contrary to every earthly outlook. From this moment life in the spirit, or by the Spirit, begins in him, with the trampling down of self-love and all the passions.
Life in the spirit, or by the Spirit, may therefore be described thus. A man is awakened by the grace of God and sanctified through the mysteries. He enters within himself and, standing before the Lord in the heart, directs all his deeds, both inward and outward, toward pleasing God. He does this through unwavering walking in His will, set forth in the word of God and received into the conscience, in hope of eternal blessedness in God, trampling down self-love and forgetting earthly interests. Such a man lives in the spirit and by the Spirit.
From this explanation the question is resolved by itself: which spirit does the Apostle mean here, ours or the Holy Spirit? He means both together. Our spirit has no power to give us spiritual life by itself. And the Spirit of God will not give it if He is not received by our spirit.
When they are joined, our spirit becomes strong by the Spirit of God, and the Spirit of God acts upon us and all that is ours through our spirit. He sanctifies and perfects everything. The one who walks in the spirit walks by the Spirit of God, but walks according to the elements of the spirit originally breathed into man.
From this explanation it is also clear what the lust of the flesh is. It is self-love with all the passions. It is called the lust of the flesh because the principal seat of self-love and the passions is in the flesh.
Thus it happened that, just as the norm of human life according to its first ordering is spiritual life in God, so when man fell, he fell into the opposite of spirit and God. He fell into fleshly life, pleasing himself with created things in this life, while forgetting God and the life beyond the grave.
When the spirit is awakened and gains strength, then self-love with the passions, or this lust of the flesh, is rejected. The law is received: never in anything to listen to the suggestions, demands, and attractions of this lust. "Let not sin reign in us, that we should obey it in its lusts" (cf. Rom 6:12-14).
The decided lot of this old man is to be crucified, so that the body of sin may be destroyed (Rom 6:6). It will end in this. But for a time the wisdom of God has allowed him to stand against the spirit, in us, yet as something alien to us, not approved, persecuted, and rejected.
By consciousness and freedom man passes over to the side of the spirit and there performs all his works. But self-love with the passions, though in him, attacks him as though from without, drawing and seducing him. The Christian's task is to attend to himself and stand in the spirit before the Lord, noticing the approaching wrongful movements and suggestions of selfhood, or the lust of the flesh, and repelling them in the name of the Lord.
This is precisely what the Apostle expressed in the words: "Walk by the Spirit, and do not fulfill the lust of the flesh." To make this lust entirely dead is God's work. Our work is not to accept any of its movements or suggestions, and however much it troubles us, to do nothing to please it.
In this consists the whole life of the Christian. It is unceasing warfare, as the Apostle says immediately after this.