What does it mean to mortify the old man in Christian life?

To mortify the old man means to actively put to death the sinful tendencies and behaviors that stem from our fallen nature. This is expressed in Colossians 3:5, which instructs Christians to mortify their members that lead to sin. The process of mortification is not about eliminating the old man completely—since it remains present until glorification—but involves continually fighting against sin through the power of the Holy Spirit and focusing on our new identity in Christ. This ongoing battle is essential for spiritual growth and maturity in the believer's life.