What does it mean to mortify sin?

Question 6060.

The mortification of sin is Paul’s blunt name for the daily work of putting sin to death, and it is the least fashionable idea in modern Christian vocabulary. We prefer to speak of growth, journey and process. Paul reaches for a word that belongs in an execution yard.

If by the Spirit you put to death the deeds of the body, you will live (Romans 8:13). That single sentence contains the whole doctrine of the mortification of sin, including who does it, how it is done and what it produces.

What the mortification of sin actually means

Mortify is simply the old English rendering of the Latin for putting to death, and it translates two Greek verbs. In Romans 8:13 Paul uses thanatoo, to put to death. In Colossians 3:5 he uses nekroo, to treat as dead or to deaden. Neither word suggests management or containment.

The Puritans made this vocabulary famous, and John Owen’s line has outlived nearly everything else he wrote. Be killing sin or it will be killing you. That is not rhetoric. It is an accurate summary of Paul’s grammar, because both verbs describe an action with a corpse as its intended outcome.

The imagery is deliberately violent, and I think that is the point. We are polite about sin in a way the New Testament never is. Paul does not ask us to discourage it, moderate it or set boundaries around it. He tells us to kill it, and a metaphor that strong is chosen because the softer ones have all been tried and have all failed us.

So the mortification of sin is not sin management. It is not learning to live alongside a besetting habit with reduced embarrassment. It is the settled intention to see the thing dead, pursued daily, in the knowledge that this side of glory the work will not be finished.

Why it is needed at all

Some Christians assume that regeneration ends the conflict. It does not. Something is decisively new at conversion, and the believer is genuinely a new creation (2 Corinthians 5:17), yet Paul writes to believers about putting sin to death precisely because sin is still there to be dealt with. You do not issue that command to a corpse.

This is why I find the language of the flesh more useful than talk of two natures at war. The flesh is the old way of operating, still resident, still capable of dominating a believer who leaves it unchallenged. The mortification of sin is the counter offensive. I have set out my thinking on that continuing struggle in why Christians still sin after being saved.

Left alone, sin does not stay at its current size. That is the whole point of Owen’s warning. Sin is never dormant. It is either dying or growing, and the believer who declares a truce in the mortification of sin has not stopped the war, only stopped fighting it.

Who does the killing

Here is the phrase that saves this doctrine from becoming grim self improvement. If by the Spirit you put to death the deeds of the body. Paul says you put them to death, and he says you do it by the Spirit. Both halves are essential, and dropping either one wrecks the mortification of sin.

Drop the Spirit and you get willpower religion, which produces either pride or despair depending on your temperament. Drop your own agency and you get passivity dressed as spirituality, waiting for a deliverance that Scripture never promises in those terms. The believer acts, and the power to act comes from the indwelling Spirit, as I have explained further in how the Spirit sanctifies us.

This is why prayer sits at the centre of the practice. Not prayer as a substitute for action, but prayer as the request for the strength in which the action is taken. Nobody has ever mortified a sin by gritting their teeth in isolation from God, and nobody has ever had it mortified for them while they carried on indulging it.

What mortification is not

It is not asceticism. Paul explicitly says that severity to the body has no value in stopping the indulgence of the flesh (Colossians 2:23). Fasting, cold showers and hair shirts can coexist happily with a proud heart. The mortification of sin goes after the desire, not the body that carries it.

It is not merit earning either. Nothing in this doctrine touches your standing before God. You are justified by faith, once and permanently, and the mortification of sin is what a justified person does because they are already accepted rather than in order to become so. Confuse those two and you will spend your life in a treadmill.

Nor is it introspection for its own sake. There are believers who spend so long examining their own hearts that they never get round to obeying anything. Owen himself warned against a self scrutiny that never lifts its eyes to Jesus. The aim is a dead sin, not a detailed inventory of a living one.

And it is not perfectionism. Paul assumes an ongoing campaign, not a single victory that settles everything. Believers who expect one decisive experience to end the fight tend to conclude, when the old desire returns, that nothing was ever real. The mortification of sin is a long war with real ground gained.

How it is actually done

Start with the specific rather than the general. Colossians 3:5 names sexual immorality, impurity, passion, evil desire and covetousness. Paul does not tell them to be holier in the abstract; he identifies targets. A believer who says they want to grow will drift. A believer who says they intend to kill a particular habit has something to aim at.

Then trace the sin back before its final act. Every fall has a chain, and the chain is easiest to break early. James describes desire conceiving and giving birth to sin (James 1:14-15). Mortification works upstream of the birth, at the point where desire is still being entertained rather than obeyed, which is why Jesus told us to cut off whatever causes us to stumble.

Keep watch on the small compromises as well. Sin rarely announces itself with a fanfare; it arrives as a slight loosening, a joke, a second look, a delayed obedience. The believer who deals with the small things is rarely ambushed by the large ones.

Then bring the truth of the gospel to bear on the desire itself. Sin advertises. It promises comfort, relief, significance or escape. The mortification of sin involves telling the truth about the advertisement, and telling a better truth about what God has already given you in Jesus.

The positive side

Paul never leaves putting off without putting on. The same letters that command the mortification of sin command the putting on of compassion, kindness, humility, meekness and patience (Colossians 3:12). Killing sin creates space, and space does not stay empty for long.

This is where the fruit of the Spirit belongs. The believer who fights lust and fills nothing in its place will fail. The believer who fights lust and cultivates love, service, honest friendship and useful work is fighting on ground where the enemy has less to work with.

Notice too that most of what Paul tells us to put on is relational. Compassion and patience are not practised alone in a room. Much of the Christian fight against sin is won in the ordinary company of other believers, doing useful things together, which is one of the quieter arguments for taking church membership seriously.

Owen understood this. So did Paul, when he told Timothy to flee youthful passions and pursue righteousness, faith, love and peace in the same breath (2 Timothy 2:22). Flight and pursuit are one movement, and the mortification of sin is only half the sentence.

So, now what?

Name one thing. Not a category, one thing. What is the sin you would least like to see written on a screen behind you at church next Sunday? That is your assignment, and the mortification of sin begins there rather than somewhere more comfortable.

Then be realistic about the timescale. A sin that has been fed for years will not starve in a weekend, and expecting it to will only set you up for a disappointment that the sin can use against you.

Ask the Spirit for strength before you ask yourself for effort, and then bring both. Cut off the obvious feeders. Tell one trustworthy believer. Put something better in the space. Expect the fight to last longer than you would like.

And if you fall tomorrow, do not conclude that the whole thing is hopeless. Confess it, get up and carry on. The mortification of sin is the work of people who are already forgiven, which is precisely why they can afford to look their sin in the face. What will you start killing this week?

So put to death what is earthly in you: sexual immorality, impurity, passion, evil desire, and covetousness, which is idolatry.

Colossians 3:5, ESV