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 Why was David a man after God's own heart?
 1st Samuel 13:14
God is not looking for the most attractive, the smartest, or the most
talented person.  1st Samuel 16:7   God does not look on the outward appearance of a person, but HE looks on a persons heart.  1st Kings 15:3   David's heart was completely devoted to God

David proved God can use you even after you have made a mistake.  1st Kings 15:5  God knew David was a man he could count on.

When we appear before God on judgement day. Heb. 9:27
Will HE say, "This is a person after my own heart?"or
Will HE say, "Well done thy good and faithful servant?"  Mat. 25:24