How do we know mortification of sin is true?

We know that the doctrine of mortification is true because it is rooted in the believer's identity as someone who has died to sin and is raised with Christ. In Romans 6:11, Paul emphasizes that believers are to consider themselves dead to sin but alive to God. This understanding leads to the practical application of mortification in a believer's life, where each Christian is called to actively resist and put to death the sinful tendencies of the flesh, as indicated in Romans 8:13 where it states that through the Spirit we put to death the deeds of the body. The ongoing struggle against sin within every believer testifies to the truth of this doctrine as we cultivate holiness in the light of Christ's return.