1 Samuel 17:1-58
One of the most famous stories in the Bible is David and Goliath. It has become a staple story in our culture; slaying the giant, the underdog overcoming the odds, it's Hollywood stuff now. The emphasis of the story isn't that David was small, Goliath was big, and David still won.
We see a contrast between David and King Saul. Saul was also a very tall man. At his ordination as king the people observed, "he stood head and shoulders above anyone else" (1 Samuel 10:23). As the King he was really the one who should have stood up to Goliath. He was likely the biggest of all the Israelites and as God's anointed king he would have been protected by God. But Saul was a coward. He even hid when he was proclaimed king (1 Samuel 10:22). He didn't trust that God would fight for him.
When David arrived at the camp and heard Goliath defying God he was angry. He knew the God of Israel wouldn't stand for this but no one was stepping forward. He had faith that God would fight for whoever would go out and so he decided it would be him. Just as God had protected him while shepherding God would do the same. They tried to fit him in Saul's armour but Saul was above average height and David was below average. He went out with no armour because he knew armour or not, God would win.
Goliath is unimpressed with little David and his sling but David knows better. "You come to me with sword, spear, and javelin, but I come to you in the name of the Lord of Heaven's Armies - the God of the armies of Israel, whom you've defied" (1 Samuel 17:45). David sinks a stone into Goliath's head before cutting it off. God defeated Goliath through David. David's height, lack of armour, and choice of weapon all show that it was God. That stone shouldn't have been able to sink into Goliath's head. It should have bounced off and maybe sting a little but it shouldn't have killed him. God made it so and fought for Israel because of the faith of David. Just as the sins of Eli's sons had caused many to die; the faith of David saved many.
In King David Israel finally had a strong, godly leader. He wasn't perfect but when he did fail he repented and sought God's forgiveness, something the judges and King Saul never did. He was considered a man after God's own heart and led Israel to unprecedented prosperity and peace because he followed God with all his heart. Israel flourished under a leader who depended solely upon God.