Why is self-sanctification harmful according to Scripture?

Self-sanctification is harmful because it suggests that individuals can contribute to their own holiness through their works, violating the truth of the gospel. According to Galatians 2:21, if righteousness comes by the law, then Christ is dead in vain, meaning that to rely on self-effort for sanctification denies the sufficiency of Christ's sacrifice. This reliance on works ultimately leads to hypocrisy, as believers who think they can sanctify themselves begin to look to the law instead of Christ. The gospel teaches that true sanctification is solely the work of Christ in the believer, who empowers them to live in holiness and keeps them separated unto Himself.