1) David served God's purpose in his generation by working humbly as a shepherd, fighting bravely against enemies of Israel like Goliath, waiting patiently for God to fulfill his promise of kingship, and leaving a godly legacy through his writings in the book of Psalms.
2) All Christians should seek to understand God's unique purpose and plan for their lives and serve Him faithfully in both their daily choices and through helping others learn about the gospel.
3) While God forgives sins, disobedience still has consequences; David's life provides an example of both God's forgiveness and the repercussions of wrongdoing.