The Shepherd

A youth with a sling and five smooth stones,
Stepped out to face giants and shatter thrones.
He played his harp to soothe a king's dread,
Then wore the heavy crown instead.
He sang out his psalms in the dark of the cave,
A leader flawed, but fiercely brave.
From Bethlehem's fields to Jerusalem's height,
His lineage carried the promise of light.

Who am i?