What does the Bible say about mourning for sin?

In Ezekiel 9:4, God directs a messenger to mark those who 'sigh and cry for all the abominations that be done' in the land. This indicates that a true mark of God’s people is their heartfelt sorrow for sin, both within themselves and in the culture around them. When a believer genuinely understands their sinful nature and the consequences of sin, it leads to deep contrition and an earnest desire for holiness. This response showcases a fundamental transformation in the heart, facilitated by divine grace, leading them to mourn over their sinfulness, as exemplified in Daniel's prayer of confession in Daniel 9.