The Mortification of Sin is a small, intense book by the Puritan theologian John Owen about how a Christian actually fights the sin that remains in them after conversion. It began as sermons to students at Oxford, where Owen was vice-chancellor, and it was published in 1656. Its most famous sentence — “be killing sin or it will be killing you” — has outlived almost everything else Owen wrote, and this is the book it comes from. It is demanding, unfashionably honest, and, for people who are serious about holiness, one of the most useful books in English.

This guide puts Owen’s argument into plain English: what mortification is, why he insists the Spirit does the work, the practical steps he gives, the errors he warns against, and how to read his famously dense prose. The full text is free in the Ochorus library.

The text and the claim

The whole book is an unfolding of one verse, Romans 8:13: “If ye through the Spirit do mortify the deeds of the body, ye shall live.” Owen reads every word of it. If — this is conditional, a duty laid on believers. Ye — believers, the best of them, not just new converts. Through the Spirit — the power is not our own. Mortify — put to death, not merely manage. The deeds of the body — the sins that still stir in us. Ye shall live — the outcome is life, vigour, assurance, communion with God.

From this he draws the sentence students have quoted ever since. Owen is addressing Christians who imagine that sin, once forgiven, can be left to quiet down on its own. No, he says:

Do you mortify; do you make it your daily work; be always at it whilst you live; cease not a day from this work; be killing sin or it will be killing you.

John Owen, The Mortification of Sin

The two options are the whole book. There is no third. Sin is not static; it is either dying or growing. Galatians 5:17 describes the war — the flesh lusting against the Spirit — and Owen’s point is that in this war a truce is impossible. What you do not kill is killing you.

Whose work it is

Owen’s first great safeguard is that mortification is the Spirit’s work, not raw willpower. He spends an early chapter insisting on it, because he had seen the alternative and knew where it led. People who try to kill sin by sheer effort do one of two things: they fail and despair, or they succeed at the surface and grow proud, having only swapped a visible sin for an invisible one. Owen calls the counterfeit versions out by name — self-punishment, forcing yourself into a temporary discipline, papering over the symptom while the root grows. Zechariah 4:6 is behind his whole method: not by might, nor by power, but by my Spirit.

But — and this is the balance that makes Owen so useful — the Spirit works through our effort, not instead of it. He does not kill sin while we sleep. He works in us so that we work, which is Philippians 2:12–13 exactly: work out your own salvation, for it is God who works in you. Owen holds both hands of that verse at once. You must fight with everything you have; and the fight is only won because God is fighting in you. Drop either half and the method collapses.

Owen’s practical steps

The back half of the book is a set of directions, and this is where readers find help they can use. A summary of the main ones:

  • Aim at the root, not just the fruit. Mortifying one visible sin while leaving the heart untouched is useless; Owen insists on “universal sincerity,” a war on all sin, because a person who spares one lust is not really at war with any.
  • Get a clear sight of the sin’s guilt, danger, and evil. Much sin survives because we have never looked hard at what it actually is and where it actually goes. Owen makes the reader stare at it until it loses its glamour.
  • Load your conscience with the guilt of it — not to wallow, but to feel the weight, and bring that weight to God rather than excusing it away.
  • Long for deliverance, and expect it. A settled, restless desire to be free is itself part of the cure.
  • Watch the occasions. Know the times, places, and moods when the sin rises, and guard them, as Jesus said in Matthew 26:41: watch and pray, that ye enter not into temptation.
  • Meditate on the greatness of God. A soul filled with a sense of God’s majesty has less room for sin’s smallness.

But every step is preliminary to the last, which is the point of the whole book. All the self-examination in the world cannot kill a single sin; only Christ can. So Owen’s final direction gathers up the rest:

Set faith at work on Christ for the killing of thy sin. His blood is the great sovereign remedy for sin-sick souls.

John Owen, The Mortification of Sin

This is the difference between Owen and mere moralism. The examination is to drive you to Christ, not to make you your own saviour. The power to put sin to death comes by faith, from the cross, applied by the Spirit. 1 John 1:7 is the ground: the blood of Jesus his Son cleanses us from all sin.

What Owen warns against

Speaking peace to yourself before God speaks it. One of the most piercing chapters warns against hushing a guilty conscience with a quick, cheap comfort that God has not actually given. Real peace comes from real dealing with sin at the cross, not from telling yourself it is fine.

Mistaking the fading of a sin for its death. A sin can go quiet because age, circumstance, or a new interest crowded it out, and the person thinks they have conquered it when they have only been distracted. Owen wants sin killed, not merely sleeping.

Fighting sin without the gospel. This is the error the whole book exists to prevent. Trying to mortify sin apart from faith in Christ is, Owen says, the very thing that drives people to monasteries and self-invented severities — Colossians 2:23 calls it a will-worship that has no power against the flesh. The gospel is not the reward at the end of the fight; it is the weapon in the fight.

Why the book is still needed

The temptation Owen wrote against is alive in a new form. Much modern Christian talk about sin has quietly grown soft — sin is reframed as brokenness to be managed, a wound to be soothed, a stage to be grown out of, anything but an enemy to be killed. Owen’s bluntness is a corrective precisely because it refuses that softness. Sin, he insists, is not a condition you live alongside; it is a force actively working your ruin, and the only safe posture toward it is war. Colossians 3:5 gives the same command in a single word — mortify, put to death — and Owen simply takes it literally.

But he is not harsh, and this is the surprise for people who expect Puritan gloom. The aim of the whole book is not misery but life and freedom: “ye shall live,” the second half of his text. A Christian who never fights sin is not a happier Christian; they are a duller, weaker, more anxious one, their assurance always leaking away because unmortified sin darkens the soul’s sense of God. The people who take Owen’s medicine are the ones who end up most free, most joyful, and most sure of God’s love — because the fight, rightly fought, drives them again and again to Christ, and it is there, John 8:36 says, that the Son makes them free indeed.

How to read it today

Read slowly, and expect to be searched. This is not a book to skim. Owen’s sentences are long and his logic is tight; read a few pages at a time and let each land.

Read it with one sin in view. The book comes alive when you apply it to a particular sin you are actually fighting, rather than to sin in the abstract. Owen wrote it to be used, not admired.

Hold the two halves together. Effort and grace, watchfulness and faith. If you come away thinking it is all your striving, or all God’s doing while you wait passively, you have read half the book.

Read Holy in Christ next. Andrew Murray’s book takes the same subject — practical holiness — into a warmer key, and the two balance each other: Owen the surgeon, Murray the shepherd.

Read Grace Abounding beside it. Bunyan’s account of his own long war with sin and doubt is Owen’s doctrine lived out in one man, and it shows that the fight Owen describes is survivable and worth it.

Owen’s book leaves you with a single conviction: that the Christian life is not a truce with sin but a war on it, fought daily, in the Spirit’s strength, by faith in a crucified Saviour. Romans 6:11 is the standing order — reckon yourselves dead indeed unto sin, but alive unto God. Owen simply shows you how to obey it, one day at a time, until the last enemy is dead.