Setting One's Mind on the Spirit
"Those who live according to the flesh set their minds on the things of the flesh, but those who live according to the Spirit set their minds on the things of the Spirit. For to set the mind on the flesh is death, but to set the mind on the Spirit is life and peace" (vv. 5-6).
Humanity's fundamental problem is its estrangement from God, which results in eternal death if it is not rectified (Gen. 2:15-17; Rom. 6:23; Eph. 2:11-12). Condemnation is the only end for those who do not have their sins covered, but for those who have been justified in Christ, there is no condemnation (Rom. 1:18-5:1). This state of being free from the Lord's condemnation is permanent and perpetual—there is no condemnation for those in Christ Jesus, for in Him we are set free from the law of sin and death (8:1-2).
God secured this release by succeeding where His law failed. His law, while good in itself, cannot solve the problem of sin because it cannot give sinners what they need to be righteous before Him or free them from sin and death (7:7-25). Another way had to be found, one that is not dependent on our law-keeping and that does not arbitrarily set aside God's standards. When the Lord condemned sin in the sinless flesh of His Son, He did what His law in the possession of sinful flesh could not do (8:3). Jesus satisfied the curse of the law in our behalf, so we need fear it no longer. He bore our penalty, and our just Lord would never commit the great injustice of making those in Christ serve a sentence He had completed for us.
But God did not only condemn sin in order that justice could be done in our salvation. He also provided, in Christ's atonement, a way for us to fulfill the law as we walk in the Spirit (v. 4). The sense here is likely twofold. First, the law is fulfilled in us via imputation. The condemnation and elimination of our sin cleaned the slate, so to speak, for Christ's righteousness to be imputed to us (2 Cor. 5:21). Legally speaking, God sees us as law-keepers—He sees that His law has been fulfilled in us—because when He looks at us, He sees Christ. Jesus provides in our justification the perfection that God requires to live in His presence, and this perfection can be credited to us only if we first admit that we cannot achieve it.
Our walking in the Spirit is evidence that we have been justified, so we must also see Romans 8:4 as referring to an experiential fulfillment of the law in Christians. That is developed more in verses 5-6, where we see that setting our minds on the Spirit brings life and peace. In bearing the curse of God's law, Christ provided for the Spirit of God to be poured out and to dwell in us (Gal. 3:13-14). Since we have been counted righteous before the Lord in Christ, the Holy Spirit can live within us and empower us to set our minds continually on the Spirit, and we enjoy His life and peace even now.
Coram Deo
Even though the Spirit of God provides us with real power to obey God now, that power does not perfect us in this life. John Calvin comments on Romans 8:1-2: "Wherever the real fear of God is vigorous, it takes away from the flesh its sovereignty, though it does not abolish all its corruptions." We should have a realistic expectation of how far we will progress in the Spirit, but not a defeatist attitude. By faith and through the Spirit, we will grow in conformity to Christ.