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By any earthy standard, the outcome of this battle would be "sure thing." However, when God enters the picture the entire outcome becomes quite different because of faith. It is interesting that David chose five smooth stones from the brook when only one was needed. Why did David do this? There is no reason given in Scripture. Consequently, whatever answer would be given would be purely conjecture.

The important aspect is that when it came time to fight Goliath, David took but one stone from his shepherdís bag (1 Samuel 17:49). There in lies the true test of faith. When it came time to act according to faith, David took but one stone rather than having several in his hand to re-load if he missed. As you read the story, this remarkable event becomes an extraordinary expression of faith because David attacked Goliath. He hurled the stone while at a full run! Judges 20:16 tells us that Israel slingers were extremely accurate. Think of the logistics of aiming and throwing a stone while both parties are rapidly approaching each other. By faith, David knew one stone was all he needed for Goliath. James tells that "faith without works is dead, being alone" (2:17-18). David showed his faith by using but one stone when he had a bag full of stones. Why the other four stones? David told Goliath,
1 Sam. 17:46-47
46* "This day the LORD will deliver you up into my hands, and I will strike you down and remove your head from you. And I will give the dead bodies of the army of the Philistines this day to the birds of the sky and the wild beasts of the earth, that all the earth may know that there is a God in Israel, 47* and that all this assembly may know that the LORD does not deliver by sword or by spear; for the battle is the LORDíS and He will give you into our hands."
David took four extra stones to either attack or defend himself while the Israelite army joined the battle. This seems more reasonable, but please understand that even this is conjecture. Thanks to David L. Baker for this material,
We have probably all grown up hearing the story of David and Goliath and how Davidís faith allowed him to slay the Philistine giant but there is more to this story than meets the eye and one minor detail is often overlooked... We read in 1 Samuel 17:40 that David picked up and hand chose five smooth stones for his sling to use as ammunition to kill the giant.
Why did David need five stones? Why not just one? One cannot help but wonder, what was going through his mind as he selected those five stones. How much faith did David really have? David knew that God was going to deliver Goliath into his hand.... or, did he? Was he thinking that he might miss with his sling shot and have to try again? He only had one chance, David knew this. We all know the outcome of the story, David killed Goliath with one stone and one throw of his sling shot but there is a reason for the other four stones and why David picked five stones specifically.
Take a look at the old testament