What is mortification of the flesh or mortification of sin?

from the old testament

• The Old Testament doesn’t use the term “mortification,” but it consistently emphasizes the need to turn from sin and live in obedience to God. For example, Deuteronomy 30:15–16 contrasts blessing and life when obeying God with death when turning to sin, showing that rejecting evil leads to true life.
• Psalm 119 repeatedly exhorts believers to follow God’s statutes and avoid sin, emphasizing that meditating on His Word strengthens resistance to wrongdoing.
• – Proverbs 25:28 warns that lacking self-control is like a city without walls, highlighting the danger of letting sinful desires rule, a principle echoed in mortification where believers “put to death” fleshly inclinations.

from the new testament

• Two passages in the King James Version preserve the word “mortification” to mean “put to death” in reference to sin: “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), and, “Mortify therefore your members which are upon the earth; fornication, uncleanness, inordinate affection, evil concupiscence, and covetousness, which is idolatry” (Colossians 3:5). Modern translations render “mortify” as “put to death,” but the idea of killing sin remains the same.
• Paul explained the need for mortification by describing the conflict between the flesh and the Spirit. Though believers are no longer slaves to sin, the flesh still loves sin and pulls against the believer strongly, producing desires that oppose God’s desires (Romans 8:5–7; Galatians 5:17). Paul described his own inward struggle in Romans 7, where he acknowledged that sinful desires remain active even in those who belong to Christ. Yet he also celebrated that believers are no longer condemned and have been set free from sin’s ruling power through Jesus (Romans 7:24–25; Romans 8:1–2).
• Jesus contrasted the life of the flesh now on Earth with the eternal perspective believers should have: “Whoever loves his life loses it, and whoever hates his life in this world will keep it for eternal life” (John 12:24–25).
• Because of this ongoing battle, Christians are repeatedly warned to take active, Spirit-dependent steps to “put to death” their sinful practices (Romans 8:13; Colossians 3:5). Paul described “those who belong to Christ Jesus” as ones who “have crucified the flesh with its passions and desires” (Galatians 5:24).
• No one will ever be perfect in this life (1 John 1:8, 10). But believers should aim to be like Jesus (1 John 3:2–3).

implications for today

Drifting in a boat is therapeutic. But drifting through our Christian life is not. Sin is ever present. “Mortification” reminds us to be on guard against it. We can’t just wish sin away, and the Spirit doesn’t change us while we stand still. God has given us the very real responsibility to obey Him and has given us everything we need to accomplish that. The Holy Spirit lives within all believers, helps us recognize our sin and strengthens us to overcome it. We aren’t in this battle alone.

Because of the Spirit’s work, we can take real steps toward holiness—such as turning away from old habits that once felt irresistible and choosing what honors Christ. Every small act of obedience—every time you say no to temptation, every prayer for help, every confession that brings sin into the light—shows that God is transforming your heart to be like Christ’s.

The work is slow at times, the struggle is real, and you may fall along the way. We are all works in progress. But God has promised to complete what He started (Philippians 1:6), so we have assurance knowing that we will make it to the end.