Mortification has several meanings in English. One is “a feeling of shame or embarrassment.” Another meaning has to do with death. The word mortify means “to kill or subdue.” In a biblical context, to mortify is to subdue the body (or its needs and desires) through self-denial and discipline (e.g., mortification of sin / the flesh). We get our English words mortuary and mortician from the same Latin root that gives us mortify. Therefore, mortification of sin / the flesh is the “killing” of sin and the flesh.
We find the word mortify in the King James translation of two passages: Romans 8:13 and Colossians 3:5.
• “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 8:13)
• “Mortify therefore your members which are upon the earth; fornication, uncleanness, inordinate affection, evil concupiscence, and covetousness, which is idolatry” (Colossians 3:5)
More modern English translations, such as the ESV or NIV, translate “mortify” as “put to death.” In both cases, the apostle Paul is exhorting his readers to “put to death” the “deeds of the body” or “what is earthly in you.” So the mortification of sin / the flesh is the putting to death or subduing of the sinful nature that still resides in believers. Let’s look at each of these passages in more depth.
“For if you live according to the flesh you will die, but if by the Spirit you put to death the deeds of the body, you will live” (Romans 8:13). Paul here is making a contrast between believers and non-believers. Non-believers are those who “live according to the flesh.” By contrast, believers are those who “by the Spirit put to death the deeds of the body.” Often in the writings of Paul, flesh and spirit are contrasted. To live “according to the flesh” is to have your mind set on the flesh and to have a mind that is hostile to God and does not submit to His law. This, in a nutshell, is the life of an unbeliever. Romans 1:18—3:20 is a vivid description of what “living according to the flesh” looks like. The person who lives his life according to the flesh will die. This is not speaking of physical death because that is the fate of all people as a result of sin. Paul is speaking of eternal death in hell.
The believer, on the other hand, by the Spirit puts to death the deeds of the body. In other words, through the working of God’s Holy Spirit, who only dwells in believers, the believer engages in the process of sanctification or growing in holiness. The believer, in contrast to the non-believer, has his mind set on the Spirit and submits to God’s law. It is important to note that the believer is not completely free from sin. Putting to death the deeds of the body is a continual process that the believer must engage in on a daily basis. The point is that one of the marks of a true believer is that he is daily putting to death the deeds of the body. This person, the one who puts to death, or mortifies, the deeds of the body and its sinful nature will live. Again, this is a reference to eternal life, or heaven.
“Put to death therefore what is earthly in you: sexual immorality, impurity, passion, evil desire, and covetousness, which is idolatry” (Colossians 3:5). Looking at the context of this passage and noting the word therefore, we must look at what preceded this verse. In verses 1–4, Paul exhorts the Colossians to seek the things that are above and to set their minds on heavenly things. Why? For we have died, not physically, but figuratively to sin and the old life, and our lives are now “hidden with Christ in God” (verse 3). If this is the case, then what are we to do? The only way the believer, whose life is hidden with Christ, can be heavenly minded is if he puts to death those things that are earthly in him. Again, we should not understand the contrast between heavenly and earthly as to mean we should have our heads in the clouds and our feet off the ground; as the expression goes, “He is so heavenly minded that he is no earthly good.” It is similar to the spirit/flesh contrast Paul made in Romans, with flesh being understood as “sinful nature.”
What is it that Paul wants us to put to death? Sinful desires and actions such as sexual immorality, impurity, passion, evil desire and covetousness, which is equated with idolatry. The person whose life is characterized by these things is said to have the wrath of God upon him (verse 6). Paul notes that this was the way of life of his readers (verse 7). All believers were at one time unbelievers and were under the wrath of God. This is a similar argument to the one Paul makes in 1 Corinthians 6:9–11 when he says the unrighteous will not inherit the kingdom of God. In verse 11, he says, “And such were some of you.” In other words, you were like that; now you’re like this. You were walking according to the flesh; now you’re in the Spirit. Because of this, put to death those things that were of the flesh. Grow in holiness and sanctification.
So what is mortification of sin / the flesh? In a word, it’s sanctification, the process by which the Holy Spirit works in the lives of God’s adopted children to grow them and form them into the image of God’s one and only Son, Jesus Christ. Sanctification is God’s will for our lives (1 Thessalonians 4:3), and it is the purpose to which He has called us (Romans 8:29).