Putting sin to death, likewise, is a difficult duty for the Christian, but Paul teaches us that it is necessary and possible. Romans 8:12–13 states that life in the Spirit includes liberation and mortification. We are no longer debtors to this present, evil age, so we walk in a different direction, think after the realm of Christ, and relate to God in a covenant of grace. Since God has delivered us, we are indebted and enabled to walk according to the Spirit, and we express our indebtedness by putting sin to death in the power of the Spirit. 

Squatters are notoriously difficult to evict. They may disappear for a while, making you think they are gone, yet they always find a way back and may even bring a few friends along. Indwelling sin is not so easy to evict either, but it is a necessary duty of the Christian life. We are Christ’s temple, therefore all squatters must go. In Romans 8:12–13, Paul teaches us that mortification is not only necessary but possible. Putting sin to death is necessary because Christ has delivered us from this sinful age and possible because the Spirit of Christ leads us in holiness. This means that we are no longer debtors to live according to the flesh; rather, we are indebted and enabled to live according to the Spirit.
No Longer Debtors
Paul concludes the first eleven verses with a negative implication: “So then, brothers, we are debtors, not to the flesh, to live according to the flesh” (Rom 8:12). You are no longer a debtor to the powers of this present, evil age because the Spirit of life has set you free from the condemnation of the moral law (v. 1) and the law of sin and death (v. 2). When the Holy Spirit takes up residence in your heart, he changes the direction of your life, the object of your mind, and your covenant with God.
Prior to regeneration, you were a debtor to the flesh. You walked according to the flesh (v. 4), set your mind on the things of the flesh (v. 5), and lived at enmity with God (v. 7). Outside of Christ you were spiritually dead, but you were by no means static or lifeless. To the contrary, you walked in the same direction as the world and followed the powers of this present age (Eph 2:1–2). Like those floating down the lazy river in an amusement park, so you were floating along in the same direction as this rebellious age. Furthermore, you set your mind on the things of the flesh. Your thinking was restricted to this sinful age, not the spiritual age of Christ. This means that you gave little thought to the divine, heavenly realm, to Christ and his gospel, or to God and his Word. Perhaps you did think about heaven–a life of eternal bliss and freedom from pain–but you did not think about Christ, your Savior and Lord. You were captivated and lured by the sins of the age. Finally, verse 7 teaches that you not only walked with the world, thought with the world, but you were at enmity with God. Before the Spirit of life united you to Christ you were in Adam, your federal head, and related to God through a broken covenant of works.
Now that you are indwelt by the Spirit of life, you are no longer a debtor to this age. You do not have to live for the flesh. This is the emphasis of Paul’s “therefore” in verse 12. Since you are united to Christ and indwelt by the Spirit, you are no longer a debtor to this world; rather, you are a son of God and heir with Christ (v. 15–17). When the world demands your allegiance or worship, you are free to refuse because you belong to another master. You were delivered from this world and bought by the Son. While we live in this world, we do not live for this world. We walk in this world, but we do not walk in sync with this world. Since we are sons of God, and no longer debtors to this age, we walk upstream in a downstream world, set our minds on the permanent age of the future, not the transient age of the present, and relate to God in a fulfilled covenant of grace, not a broken covenant of works.
Now, Paul is not just saying you do not have to live according to the flesh. In verse 13, he emphasizes that you ought not to do so. The transition from v. 12 to v. 13 is a shift from deliverance to warning. In verse 12, Paul emphasizes that you do not have to live for this world because you have been delivered from its grip. In verse 13, Paul shifts to a warning, saying that you ought not to live according to the flesh because those who do will surely die.
Verse 13 “If you live according to the flesh, you will die.” The first thing to note about this conditional warning is what it is not saying. It is not saying that those who have been delivered from this present age can lose their salvation. Rather this is a warning to the unrepentant, those who hold Christ in one hand and their sin in the other. Death is the eternal punishment for those who willfully persist in unrepentant sin, who obstinately refuse to part with their beloved sins, and who claim God as their Savior but deny him as their Lord. Death is the just punishment for those who walk according to the flesh, set their minds on the things of the flesh, and live according to the flesh. 
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We enter into a new life by the Spirit, a new spiritual life, in which sin no longer reigns over us. Because our sins have been dealt with and our old man has been crucified with Christ, the barrier that existed between us and God has been removed and we now receive the gift of the Spirit. And by the power of the Spirit we are enabled to walk in newness of life.

In one sense, we rightly think of sanctification as a progressive work. As the 1689 LBCF states, this is the Spirit’s work of destroying “the whole body of sin” and strengthening “all saving graces, to the practice of all true holiness” (13.1). When we encounter hagios/hagiazo  (ἁγίος / ἁγιάζω) in NT usage, however, we also find a definitive aspect in the way the words are used.
We can also push a bit further by exploring how the New Testament describes what happens in conversion. There are several passages we could look at, but let us draw our attention to the one that is probably most familiar, Romans 6:1-14:
What shall we say then? Shall we continue in sin that grace may abound? Certainly not! How shall we who died to sin live any longer in it? Or do you not know that as many of us as were baptized into Christ Jesus were baptized into His death? Therefore we were buried with Him through baptism into death, that just as Christ was raised from the dead by the glory of the Father, even so we also should walk in newness of life. For if we have been united together in the likeness of His death, certainly we also shall be in the likeness of His resurrection, knowing this, that our old man was crucified with Him, that the body of sin might be done away with, that we should no longer be slaves of sin. For he who has died has been freed from sin. Now if we died with Christ, we believe that we shall also live with Him, knowing that Christ, having been raised from the dead, dies no more. Death no longer has dominion over Him. For the death that He died, He died to sin once for all; but the life that He lives, He lives to God. Likewise you also, reckon yourselves to be dead indeed to sin, but alive to God in Christ Jesus our Lord. Therefore do not let sin reign in your mortal body, that you should obey it in its lusts. And do not present your members as instruments of unrighteousness to sin, but present yourselves to God as being alive from the dead, and your members as instruments of righteousness to God. For sin shall not have dominion over you, for you are not under law but under grace.
Dead to Sin, Alive to God
Probably no passage is more instructive when it comes to definitive sanctification than this text. The constraints of this post will not allow me to give a full and detailed exposition of it, but here are the main lines of thought. Paul has just demonstrated in Romans 3:21-5:21 that the believer’s righteous standing and acceptance with God is not based on his own works but on the work of another on his behalf, even the redemptive work of Christ. He has been setting forth the glorious doctrine of justification by grace alone through faith alone in Christ alone. But now in Romans 6:1, he anticipates an objection to this doctrine and a potential abuse of it by wicked men: “But Paul, if, as you say, sinners as sinners are justified by grace alone through faith alone, why not just keep on living in sin that grace may abound? It doesn’t matter how we live.” This is the error and the objection Paul is anticipating as he begins this chapter.
He writes in verse 1, “What shall we say then? Shall we continue in sin that grace may abound?” Having anticipated the objection, he then answers the objection: “Certainly not! How shall we who died to sin live any longer in it?” We have an aorist in the indicative mood, which normally points to a past time event. There was a specific point in the past when this death occurred.
Paul next goes on to give an extended explanation in vv. 3-10. He explains that the believer died to sin in the death of the Lord Jesus. We who are in Christ are united to Him in His death to sin, and we are also raised with Him in His resurrection to live a new life. This is symbolized by our baptism.
When did this happen? In one sense, we died with Christ when He died. Jesus was dying as our substitute and representative, even before we existed. In fact, we were chosen in Him before the foundation of the world, (Eph. 1:4). But we do not actually die with Him in our legal position and standing before God until our conversion. Our old man was crucified with Him as a completed past action in that very moment that we were joined to Him by faith.
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If God entered our world as a baby in Bethlehem, then God must have made our world for the purpose of becoming a baby in Bethlehem. And thus it demands that we recognise that humanity is not a self-defining, self-advancing, self-worshipping species; or at least, we are not designed to be, and the attempt is doomed to failure. For we were made by God so that God could come to live with us. Christ came down to earth from his heavenly throne in order to bring us back up with him. To make us truly human by turning us humans back to our creator.  

The incarnation, celebrated by Christians at Christmas, was described by CS Lewis as not only the central thing about Christianity, but, if true, “the central event in the history of the Earth — the very thing that the whole story has been about”. It is perhaps for this reason that one hears so little about it, even at Christmas. It is far easier to talk platitudes about a “season of goodwill” than think about something which, if it is worth talking about at all, must redefine our entire understanding of reality.
But our moment in history, as we end 2021, demands that we must think about it. Many of the social malaises currently afflicting us arise from abandoning the Christianity which provided the structural timbers of Western culture, central to which is the Incarnation. If the timbers of the house are allowed to rot, it is only a matter of time before the roof falls in.
Let’s start with what this “central event in the history of the Earth” is. “The Word became flesh and made his dwelling among us”, says the Apostle John. God became man. The creator became part of his creation. In-carn-ation means “becoming flesh” (it’s the same root as “carn-ivore”, a creature that eats flesh); the infinite, eternal God the Son became blood, brain, muscle and bone. He became what he was not without ceasing to be what he eternally was. And so Mary’s Son is simultaneously the infinite, creator God, higher and greater than us in ways we simply cannot conceive, and normal, finite man, like us in every way — except without sin.
Now if this is new to you, you will find it almost unbelievably strange. It is a mystery which the human mind cannot fathom. And yet, strange as it is, it is this teaching which (along with Jesus’ death and resurrection) has given birth to some of the most precious and revolutionary ideas which have been foundational for Western society; and which, as Christian influence wanes, we are now seeing crumble.
The first of these ideas is that it gives huge value to our bodies. Most people in the Greek-influenced ancient world thought of the body as a mere container or, worse, an unfortunate prison for the soul. But if God the Son has deliberately taken to himself a human soul and body, then the very opposite must be true. Christ’s body is now as integral to his existence as his deity. And as we start 2022, technology seems to be more and more alienating us again from our bodies. The “metaverse”, whatever it is, promises almost total liberation from actual physical existence. Countless schoolchildren are being constantly told that their body is alien to, and perhaps the enemy of, who they really are. So the value of our bodies, and their essential importance to our being, is a message that is desperately needed.
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