Slaying the Giant 

David had no doubt in his mind that it had been the Lord who had given him victory over the wild animals that attacked his flocks (verses 34-37). David had gained the confidence to face Goliath: "He [the Lord] will deliver me out of the hand of this Philistine." And: "The battle is the LORD's" (verse 47).

Besides, in his spare time while tending the sheep, David had probably sharpened his aim by practicing with his sling for hours on end, like boys today knocking tin cans off of fence posts, till he knew he wouldn't miss. "A sling was the typical equipment of a shepherd. It was a hollow pocket of leather attached to two cords. Putting a stone in the pouch, the slinger would whirl it around his head to build up momentum. Releasing one of the cords would hurl the stone at its target. Slingers were a regular part of armies in the ancient Middle East (see Judg. 20:16)" (Nelson Study Bible, note on 17:40).

It didn't matter that Saul's armor didn't fit because David knew that the entire assembly of soldiers would be shown that the Almighty, All-Powerful Lord of Hosts saves not with sword and spear (verse 47). David exhibits a remarkable faith and courage for one so young.

When David slays Goliath, the Philistines flee (verse 51), breaking their original agreement that, if their champion were defeated, they would accept servitude to the Israelites (compare verse 9). We might wonder as to whether the Philistines had originally intended to be bound by this agreement. More likely, considering the stature of Goliath, they probably had not even considered the possibility that he could be defeated. In any case, we do not find the Philistines being subservient even after they get over the shock of losing. Instead, they remain as Israel's enemies. In the wake of David's victory, Saul asks whose son he is. The Nelson Study Biblecomments: "How does this question fit with the fact that David had been serving as a musician in Saul's court (16:18-23)? Saul's unstable mental condition (16:14, 15) may have affected his memory. Saul may have recognized David as his court musician but forgotten the name of David's father. He would need to know it in order to reward David's family (v. 25). It is also possible that in his question, Saul's principle interest was not David's identity, but the possibility that David was a contender for the throne of Israel" (note on 17:55).