Romans 8:12-13 — 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.
In Romans 7:4, "mortify" (put to death) is the act of God through the death of Christ. The same word here is the believer's responsibility, but the power is from the Holy Spirit.
We owe nothing to our flesh. It brings death — and so we are under no obligation to it. Nor can we live with an "I can't help it" attitude to sin.
I cannot stress too strongly that this portion of the epistle is in a stream of teaching that is addressed to those who have been born again, have been made alive in Christ, and have gone on with God to spiritual knowledge. There have been commentators who have taken the next portion of our text and twisted it far from its possible meaning. We now read, "For if ye live after the flesh, ye shall die." Those who have attempted to refer this passage to the second death nullify all that has gone before concerning the nature and certainty of justification by faith apart from the works of the law. The death that is mentioned here cannot, in any exegesis that recognizes the advancing nature of the revelation of truth in the epistle, be interpreted as referring to the second death. Paul has little more than finished the declaration that "there is therefore now no condemnation to them that are in Christ Jesus." And are not the objects of this revelation here addressed as "brethren"? To interpret this passage as referring to the second death would make it read, "Brethren, if ye live after the flesh ye shall not be brethren any more"; or, "Possessors of eternal life, if you live after the flesh, your eternal life will shrink to six-months life, or ten-year life." Or it would be made to read, "Justified ones, you shall become unjustified"; "born ones, you shall be unborn." To state the matter thus is to refute the idea. The death that is mentioned here is not separation from God for eternity. — Donald Grey Barnhouse, God's Heirs, page 52.
We should realize that just as the verdict of condemnation doesn not make a sinner more evil, so the verdict of justification does not make the believer more righteous. — God's Heirs, page 53.