“This day the Lord will deliver you into my hand, and I will strike you down …that all this assembly may know that the Lord saves not with sword and spear. For the battle is the Lord's.” Young David’s victory over Goliath inspired “this assembly” – the Israelite soldiers who had been paralyzed by fear – to believe that God could make them victorious. David’s faith and courage drew others to him (like Jonathan, ch 18), while his growing reputation made King Saul jealous and afraid.
David’s faith, more than his skills or background, allowed God to use him. Faith keeps our focus on God, and on what He can accomplish, rather than on our own limitations.