4 that the righteous requirement of the law might be fulfilled in us who do not walk according to the flesh but according to the Spirit.
who do not walk [behave as a course of life]
The Greek grammar does not mean that the righteousness of the law might be fulfilled in us if we walk not after the flesh but after the Spirit. Rather, Paul described here a true believer’s bent. This is not a condition for walking in the Spirit. The words “who walk not after the flesh but after the Spirit” characterize those in whom the righteousness of the law is fulfilled. Those “in Christ” are on a new journey with a new capacity.
It follows the nature of the believer’s status in Christ that it transforms his behavior. Both internal attitudes and external behavior change because there is a new capacity residing in every Christian. The last part of verse four has nothing to do with justification; it simply identifies those who are justified.
Walking after the flesh carries the sense of verse five: “For those who live according to the flesh set their minds on the things of the flesh, but those who live according to the Spirit, the things of the Spirit.” These people are those who “mind” things of the flesh. The idea is not immoral conduct of believers in this verse but the way we walked as non-Christians. Attempting to establish our own righteousness as over against accepting the righteousness of Christ is the big tension between true Christianity and alien Christianity.
Self-justification is here set over against justification by faith. There are two fundamental courses of life we can take. We can trust self-righteousness or Christ’s righteousness for salvation. The law is weak through the flesh, so the law cannot save a soul.
The word “walk” has the idea of live as a course of life. It comes from two words: around and to walk, to walk around. The reference is to the whole activity of the Christian’s life. Our legal or forensic right with God is one thing, but how we apply what we have in Christ is another thing. The word “walk” is a turn to the experiential side of Christianity. It is a metaphor dealing with the new life the Holy Spirit gives the believer.
Walking here is not an admonition but a statement of fact that applies to all believers. This is true of every Christian without exception. No true Christian lives his life according to the norm of the flesh but according to the norm of the Spirit. Walking here is not a commandment but a factual declaration that relates to each believer. Every true Christian will walk in the Spirit. Our liberty in Christ allows the believer freedom to please God. The Holy Spirit produces new life in every Christian.
The Christian does not fulfill the law by walking in the Spirit, but his bent is to live the Christian life by the Spirit of God.
Obedience of believers has its root in Christ’s work on the cross. We cannot separate sanctification from justification. It is inevitable that those who are justified will live out that justification in sanctification; it is their nature to do so. This is because at our justification God gave each believer the indwelling Holy Spirit so that we will live the life we are supposed to live.