Answer: He had faith in God. He knew that God would deliver him out of the hand of Goliath. When David slain Goliath this impressed King Saul, this is because David did this with a sling and 5 smooth rocks.
But later on King Saul became jealous. Saul appointed David leader over his armies, but he became jealous and angry because of David's success and sought to kill him. David went to various places as he fled from Saul, and he and men who joined him received help from several people. In the end David still over ruled Saul.
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