The Greek verb pentheo means to mourn, grieve, or lament — often with a deeper, more settled grief than klaio (weeping). It describes the mourning of bereavement, of deep remorse, and of spiritual grief over sin. In the New Testament it appears in the Beatitudes and in the calls to mourn over sin rather than boast in it.
The second beatitude — 'Blessed are those who mourn (penthountes), for they will be comforted' (Matthew 5:4) — is one of the most challenging and paradoxical statements in Scripture. Spiritual mourning is not depression but the grief of those who see reality clearly: the reality of sin, the reality of a suffering world, the reality of the gap between what is and what God intends. Paul rebukes the Corinthians for boasting about sin when they should have been mourning (1 Corinthians 5:2). James calls believers to mourn and grieve and wail over worldliness (James 4:9). The comfort promised to the mourning is eschatological — God Himself will wipe away every tear in the new creation.