"According to the stories I always heard, little David trusted explicitly in God when he went to fight Goliath the giant. So why did he pick up five stones--did he think he'd miss?"
The passage in question is found in 1st Samuel 17. Goliath was a giant warrior who was the champion of the Philistine army. The Philistines were at war with the Israelites and Goliath daily challenged the Israelite army to a winner-take-all dual. For whatever reason, none of the Israelite soldiers felt up to the challenge. David, the youngest son of Jesse, was visiting his brothers on the front line and heard the challenge. He went to King Saul of the Israelite army and offered his services in the name of the Lord. The king reluctantly accepted and David "took his staff in his hand, and chose five smooth stones from the wadi, and put them in his shepherd's bag, in the pouch; his sling was in his hand, and he drew near to the Philistine" (1 Samuel 17.40).
So why five stones?
The answer may be a parable for the rest of us to learn from.
According to the story, David was facing a challenge such as he'd never faced before. The fate of the army of Israel, indeed, the whole of Israel, was at stake and he was going to either be the hero or the goat. Much hung in the balance of his actions.
But little David didn't trust in his own talents and gifts alone, he placed his trust in God that God would do what God could do in the given circumstances. On the other hand, it is clear David didn't just run out into the fray totally unprepared either.
First he tried on Saul's armor and hefted his sword, but they didn't fit (1 Samuel 17.38-39). The next thing he does is remove the armor, take up the tools he is most comfortable with (his staff and a sling), and then heads to the nearest stream to pick up five smooth stones for battle.
But why five? Wouldn't absolute trust in God indicate only one stone was needed? In fact, in the story it only takes one to kill the giant. So why did he take up five?
Because although David trusted in God, David also knew that to take on any challenge in life--even with God's help--he needed to be as well prepared as he could be. So he did his homework. He studied Goliath and his ways. He learned what he could about the Philistines and their methods. And he discovered something crucial to his decision to picking up five smooth stones, instead of only one.
Goliath had brothers--four of them (2 Samuel 21.19-22). David knew there could well be the possibility that when he killed Goliath that his four brothers might try to extract revenge. So David prepared himself: he picked up five stones, one for each of the Philistian brothers.
According to the account, David trusted in God to do what God could do. But he didn't just leave it all up to God. David did what David could do to insure his own competence in the face of a challenge. David picked up five stones because he'd done his homework and he was prepared. How can we do less when faced with our own challenges in life?