If we were pure of all sin, and if we burned with a perfect love toward God and our neighbor, then we would certainly be righteous and holy through love, and there would be nothing more that God could require of us. That does not happen in this present life but must be postponed until the life to come. We do indeed receive the gift and the first fruits of the Spirit here (Rom. 8:23), so that we do begin to love; but this is very feeble. If we loved God truly and perfectly, as the Law requires when it says (Deut. 6:5): “You shall love the Lord your God with all your heart, etc.,” then poverty would be as pleasant for us as riches, sorrow the same as pleasure, death the same as life. Indeed, one who loved God truly and perfectly would not be able to live very long but would soon be devoured by his love. But human nature now is so submerged in sin that it cannot think or feel anything correct about God. It does not love God; it hates Him violently. Therefore, as John says (1 John 4:10), “not that we loved God but that He loved us and sent His Son to be the expiation for our sins.” And above (2:20): “Christ loved me and gave Himself for me”; and in the fourth chapter (vv. 4–5): “God sent forth His Son, born under the Law, to redeem those, etc.” When we have been redeemed and justified through this Son, we begin to love, as Paul says in Rom. 8:3–4, “What the Law could not do, in order that the just requirement of the Law might be fulfilled in us,” that is, that it might begin to be fulfilled. Therefore what the sophists taught about the fulfillment of the Law is sheer imagination.
With the words “walk by the Spirit” Paul shows how he wants his earlier statements to be understood: “Through love be servants of one another” (5:13) and “Love is the fulfilling of the Law” (Rom. 13:10). It is as though he were saying: “When I command you to love one another, I am requiring of you that you walk by the Spirit. For I know that you will not fulfill the Law. Because sin clings to you as long as you live, it is impossible for you to fulfill the Law. But meanwhile take careful heed that you walk by the Spirit, that is, that by the Spirit you battle against the flesh and follow your spiritual desires.” Thus he has not forgotten the matter of justification. For when he commands them to walk by the Spirit, he clearly denies that works justify. It is as though he were saying: “When I speak about the fulfilling of the Law, I do not intend to say that we are justified by the Law. But what I am saying is that there are two contrary guides in you, the Spirit and the flesh. God has stirred up a conflict and fight in your body. For the Spirit struggles against the flesh, and the flesh against the Spirit. All I am requiring of you now—and, for that matter, all that you are able to produce—is that you follow the guidance of the Spirit and resist the guidance of the flesh. Obey the former, and fight against the latter! Therefore when I teach the Law and urge you on to mutual love, do not suppose that I have retracted the doctrine of faith and am now attributing justification to the Law or to love. What I mean to say is that you should walk by the Spirit and not gratify the desires of the flesh.”
Therefore Paul uses his words with precision and care, as though he were saying: “We have not yet attained the fulfillment of the Law. Consequently, we must walk and be exercised by the Spirit, so that we think, say, and do what is of the Spirit and resist what is of the flesh.” This is why he adds: “And do not gratify the desires of the flesh.” It is as though he were saying: “The desires of the flesh are not yet dead, but they always sprout up to talk back and fight back against the Spirit.” No saint has a flesh so holy that when it is offended it would not rather bite and devour or at least subtract something from the commandment of love. Even at the first impact he cannot restrain himself from irritation with his neighbor, a desire for revenge, and hatred for him as though he were an enemy—or at least less love than he should have according to this commandment. This happens even to saints.
Therefore the apostle has established this as a rule for the saints: that they should be servants of one another through love, that they should bear one another’s weaknesses and burdens (6:2), and that they should forgive one another’s trespasses (Matt. 6:12–15). Without such ἐπιείκεια it is impossible for peace and concord to exist among Christians. It is unavoidable that you are offended frequently and that you offend in turn. You see much in me that offends you; and I, in turn, see much in you that I do not like. If one does not yield to the other through love on matters like this, there will be no end to the argument, discord, rivalry, and hostility. Therefore Paul wants us to walk by the Spirit, so that we do not gratify the desires of the flesh. It is as though he were saying: “Even though you are aroused to anger or envy against an offending brother or against someone who does something unkind to you, still resist and repress these feelings through the Spirit. Bear with his weakness, and love him, in accordance with the command: ‘You shall love your neighbor as yourself.’ For your brother does not stop being your neighbor simply because he lapses or because he offends you, but that is the very time when he needs your love for him the most. The commandment ‘You shall love your neighbor’ makes the same requirement, namely, that you not submit to your flesh—which hates, bites, and devours when it is offended—but that you fight back at it by the Spirit and that through the Spirit you continue in your love for your neighbor, although you may find nothing in him that deserves your love.”