"Therefore, brethren, we are debtors, not to the flesh, to live after the flesh. 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." (Romans 8:12-13)
Hello P---! It's great to hear from you. Great question.
Those two verses, Romans 8:12-13, have troubled many Christians due to the fact that the terms "flesh" and "Spirit" have commonly been interpreted to mean the 'sinful nature' (and thus to live by the flesh is to sin) vs. the 'new nature' (and thus to live by the Spirit is to not sin); ie. the flesh wants to lust but the Spirit wants you to be chaste. Because of this Romans 8 becomes confused, especially verses 12-13, which would seem to imply that Paul finally goes back on his prior teaching of justification by grace through faith without works (chapters 1-5) and now says we must not live according to the flesh (that is, we must not sin) in order to be saved.
The first thing we come to realize is that the Scriptures do not define those terms in that way. For a brief survey of the usage of the word "flesh" see: 2 Chronicles 32:8, Jeremiah 17:5, John 1:13, 6:63, Romans 7:5, 1 Corinthians 1:26-3, 2 Corinthians 1:17, 11:17-18 (on to 12:10), Galatians 3:2-3, 4:29, Philippians 3:3-9... You'll notice that the definition 'sinful nature' does not fit, but rather the true meaning is (loosely) 'human competency, sufficiency, accomplishment, power, effort, muscle, ability, etc.'; ie. it's not about what I can do (flesh) but what God can do (Spirit). The Spirit is the opposite of the flesh, being the power, muscle, sufficiency, ability, competency of God (see Zechariah 4:6, 2 Corinthians 4:7, and an abundance of other similar passages). The ISBE (International Standard Bible Encyclopedia) calls the flesh "the capabilities of our earthly constitution" and that "the expression does not convey, as some have supposed, the idea of inherent sinfulness of the flesh." Of course, the word has other usages also, such as simply the human body, but I have never seen it refer to sinfulness.
I know that's a lot to chew on, but it is necessary to understand our terms before approaching Romans 8:12-13.
Paul's whole point in Romans chapters 6-8 is that if we live by the law it will produce the fruit of death in us, yet if we live by grace it will produce the fruit of life in us (6:14). We have been delivered from the law so that we might be enabled to bear good fruit unto God (7:4) by walking according to the new way of the Spirit (7:6) and not by the old way of the law. Having been justified by faith, we are brought into a glorious new position in Christ to walk in newness of life (6:4)! If we live by the law, failure and sin will be our experience (7:7-25). This is not because the law is bad but because we are "fleshly"; our human power is not strong enough to overcome the sin that seeks to rule in us (7:14). Therefore if we live according to our flesh, seeking to keep the law, we will experience the fruit of death.
In chapter 8 we turn to the Spirit and see what it is to walk by grace. It starts with: "There is no condemnation to those who are in Christ Jesus"! Because of Jesus' death the righteous requirements of the law are fulfilled in (not "by") us (8:3-4)! Starting from this point we now can live "under grace" (6:14) and watch the dominion of sin become powerless, because we cease trying to accomplish righteousness by our own works/flesh under the law. Resting in the truth of righteousness by grace, we find the opposite effect: the fruit of life is aroused in us (compare with the opposite effect in Romans 7:5). It is because we are living by faith out of the reality that God in Christ has done it all for us (the accomplishment of the Spirit) we are filled with the fruit of "life and peace" (8:6)!
Paul practically sums up Romans 6, 7 and 8, which chapters answers the objection, "If we are saved by grace, shall we not continue in sin?", by stating, "Therefore, brethren (speaking to Christians about how to live now that they are saved, not to unsaved people needing to be saved), we are debtors, not to the flesh, to live after the flesh." The emphasis is on the word "NOT". Brothers, we do NOT have any obligation to live after the flesh/the law! This is a declaration of freedom! We do not have to live according to our own power anymore, but we can now finally rest completely in the power of God - the finished work of Jesus Christ at the cross for us! However, that freedom is coupled with a warning and a promise. We are not under bondage to keep the law (to walk after the flesh) - but if we do seek to live after the flesh we will experience the fruit of death. But, if you walk in the new way of the Spirit you will experience the fruit of life. Paul is speaking about the experience of sanctification and the practical results of walking according to the flesh or the Spirit - not heaven and hell.
P---, I can personally attest to the truth of Romans 6:14. Since I have believed the good news that I am righteous before God by faith in Christ and not by my works, and that I am not under obligation to keep the commandments, I've experienced freedom from the dominion of sin in my life and know what it means to have life and peace. Of course, some days I wake up and my mind isn't set on that glorious truth, and those are the days that aren't so good. But it is not by feeling an obligation to the law or by striving to keep the commandments that I experience victory in the realm of sanctification; it is only when I rest in the overwhelming love of God demonstrated in finished work of Jesus Christ at the cross for me that I experience the God-intended victory of the new life in the Spirit. One way produces sin and failure, and the other way produces life and peace. I am still learning to fix my mind unwaveringly on Jesus and His grace and the wonder of justification by faith.
So this is how we live, instead of living by law:
"For I through the law am dead to the law, that I might live unto God. I am crucified with Christ: nevertheless I live; yet not I, but Christ liveth in me: and the life which I now live in the flesh I live by the faith of the Son of God, who loved me, and gave himself for me." (Galatians 2:19-20)
Living by the Spirit is resting in the finished work of Jesus Christ on cross. We live by that faith in Jesus, who loved us and gave Himself for us. Flowing out of that faith is a love back to God and a love for others for whom Christ died. It is grace, not law, that teaches us how to live, showing us what free, non-obligatory love is, and constraining us to live a life filled with the same love that we have received from God.
Therefore may God fill us all with the knowledge of His grace and love, which glory far surpasses the fading glory of the law.