Walk And Fulfill, 16-17
Paul's command to us as God's children is to the point, saying "Walk in the Spirit." He mentions this again in verse 25. We will either walk in the Spirit or fulfill the lusts of the flesh. He reminds us that the flesh lusts against the Spirit and the Spirit against the flesh, and these do not work together but are contrary to each other--they are opposites. When the flesh lusts against the Spirit, then we cannot do what we would if we did not have this hindrance.
Spirit And Law, 18-24
Those who do not have the Spirit of Christ are under the law, and they must meet the perfection of law's command, otherwise they will fall short of the required perfection of God's righteousness, the righteousness which is by faith in Christ. But if we are led by the Spirit of God, then we are not under the law, under the demands of the law for righteousness. Paul says to the Romans in Chapter 10:5 For Moses describeth the righteousness which is of the law, That the man which doeth those things shall live by them. We cannot live the righteousness of the law, for the demand is more than we can accomplish.
Paul gives a sample list of the works of the flesh: adultery, fornication, uncleanness, lasciviousness (all sorts of evil), idolatry, witchcraft, hatred, variance, emulations, wrath, strife, seditions, heresies, envyings, murders, drunkenness, revellings, and such like. This, of course, is not a complete list, but it is a sample only. These are the things that hinder a man's meeting God's holy commandments, and those who do these things will not inherit the kingdom of God.
That which is from the Spirit is not works but fruit. We are not saved by works but serve the Lord through the fruit of the Spirit. He now gives a sample list of the fruit of the Spirit: love, joy, peace, longsuffering, etc. There is no law against these things, because they are produced by the Spirit of God; they are not worked out by the flesh or by works. That is because, if we have the Spirit of God, we have crucified the flesh and are not under the weight of the flesh's affections and lusts but can overcome them by faith in Christ.
Living And Walking, 25-26
We live in the Spirit by faith in Christ. Romans 8:9 But ye are not in the flesh, but in the Spirit, if so be that the Spirit of God dwell in you. Now if any man have not the Spirit of Christ, he is none of his. The Spirit of God dwells in us and will never leave us nor forsake us. So, if we live in the Spirit, with his indwelling and his ever-presence, then let us also walk in the Spirit. The walking is the evidence of our living. It would be well for us to remember these two great verses in Philippians 2:12-13 Wherefore, my beloved, as ye have always obeyed, not as in my presence only, but now much more in my absence, work out your own salvation with fear and trembling. 13 For it is God which worketh in you both to will and to do of his good pleasure.
Our attitude is changed when we are saved by the grace of God through the working of the Spirit of God, so in verse 26 Paul exhorts us not to (1) be desirous of vain glory (arrogance), (2) provoke one another, or (3) envy one another. What good Christians we would be if we would heed Paul's ample warning about not fulfilling the lusts of the flesh but walking in the Spirit!