Romans 8:12-13
12 Therefore, brethren, we are debtors, not to the flesh, to live after the flesh.
13 For if ye live after the flesh, ye shall die: but if ye through the Spirit do mortify the deeds of the body, ye shall live.
The Grace New Testament Commentary - Romans 8:12-13
8:12-13 Paul now brings this unit of his discussion (Romans 8:1-13) to a close. We Christians (brothers) are in no way obligated to the flesh to live in relation to the flesh. But Paul is far from denying the possibility of this. In fact, he bluntly warns his Christian readers that if you live in relation to the flesh you will die.
In fact, Paul had already tasted a "death experience" in the days when he struggled unsuccessfully against his sinful impulses (see discussion Romans 7:9, 11). Thus, a kind of fellowship death had occurred, cutting him off from the experience of God's life. By contrast, if by the ministry of the Spirit we put to death the deeds of the body - that is, if we cease to obey the body's desires - then we can enjoy the eternal life that God has given to us as a free gift (cf. Romans 6:22-23).
The word translated "you will live" (zēsesthe) recalls the word zēsetai in Romans 1:17 ("Now the one who is righteous by faith shall live"). Precisely in the manner outlined in Romans 8:1-13, the justified person can, by the power of the Spirit, live righteously in his experience.
Charles Bing - Grace, Salvation, and Discipleship - Die by the Flesh or Live by the Spirit, Romans 8:13
(Salvation) Interpretation: Those who live in the flesh will go to hell because they lose their salvation or were never saved, but if the Spirit is in them, they will be saved.
(Discipleship) Interpretation: Believers who live by the flesh experience deadness in their fellowship with God, but if they live by the power of the Spirit, they will experience the fullness of God's life.
Romans 8 continues Paul's discussion of sanctification and how the believer can have victory over sin. Verse 1 states, "There is therefore now no condemnation to those who are in Christ Jesus, who do not walk according to the flesh, but according to the Spirit." Though commonly understood as a reference to eternal hell, "condemnation" in this context of sanctification (as it is also used in Rom. 5:16, 18) speaks of the tragic effects of sin on the believer's life in Christ. The second clause of verse 1 is not in many modern translations, but it has strong manuscript evidence and says the same thing as verse 4. The emphasis of this section is living according to the Spirit as opposed to living according to the flesh and the consequences of each.
In Romans 8 Paul is speaking to Christians (he calls them "brothers" in v. 12 and says they had received the Holy Spirit in v. 15) about their sanctification experience that delivers them from sin and its consequences, so we can easily see which interpretation fits best here. Living in the flesh is the choice (expressed by "if") a Christian makes when he sides with his old sinful flesh. "You will die" expresses a consequence of God's anger at sin. The significance of death here is the same as in Romans 6:23 (see the previous discussion). There will be a deadness in that Christian's experience and a separation from the vitality of divine life. On the other hand, if the believer submits to the indwelling Holy Spirit, he will "live" or experience God's invigorating and enriching life.
According to Romans 8:9, the test of salvation is not whether one lives in the Spirit but whether the Spirit lives in him: "Anyone who does not have the Spirit of Christ does not belong to him." Verses 9-11 are stated as assurances that the readers do indeed have the Holy Spirit, therefore they have the power to have victory over sin. Verses 12-13 give the readers two possibilities. To live according to the flesh is to have a fleshly mindset and to live according to the Spirit is to have a spiritual mindset. A fleshly mind-set leads to spiritual deadness, but a spiritual mindset leads to a victorious life of righteousness.
The strong assumption and designation of these readers as believers is contrary to the interpretation that they could lose their salvation or prove to have never been saved. We have shown earlier that death does not automatically refer to eternal death, but in its essence speaks of separation. In this case, the context shows that death refers to the believer's separation from God's life-giving power, thus it is (Discipleship), not (Salvation).
As believers, we can choose to live fleshly or sinful lives, but that only brings a deadening of the experience God wants for us. Better to choose to let Jesus Christ live out His life through us and experience the power of His resurrected life.