Charles Ellicott Commentary 1 Samuel 17:40

"And he took his staff in his hand, and chose him five smooth stones out of the brook, and put them in the shepherd`s bag which he had, even in his wallet; and his sling was in his hand: and he drew near to the Philistine." — 1 Samuel 17:40 (ASV)

And he took his staff in his hand. —It was a true stroke of military genius in David, this determination of his to fight only with the weapons, weak and unimportant though they seemed, with which he was familiar, and in the use of which he was so skillful; nor was the outcome of the combat, now that he had resolved to use the sling, in any doubt. It has been well said he was like one armed with a rifle, while his enemy had only a spear and a sword, and if only he could take sure aim the result was absolutely certain.

Wordsworth, again, on the words “chose him five smooth stones out of the brook,” refers to Augustine’s Commentary, who finds here a deep mystical significance. It is an admirable specimen of the Patristic School of Exposition, which, although quaint, and not infrequently “far-fetched,” will always, and with good reason, possess great power over the minds of the earnest and devout student. “So our Divine David, the Good Shepherd of Bethlehem, when He went forth at the temptation to meet Satan—our spiritual Goliath—chose five stones out of the brook. He took the five books of Moses out of the flowing stream of Judaism.

“He took what was solid out of what was fluid. He took what was permanent out of what was transitory. He took what was moral and perpetual out of what was ceremonial and temporary. He took stones out of a brook, and with one of these He overthrew Satan. All Christ’s answers to the tempter are moral precepts, taken from one Book of the Law (Deuteronomy), and He prefaced His replies with the same words, ‘It is written;’ and with this sling and stone of Scripture He laid our Goliath low, and He has taught us by His example how we may also vanquish the tempter.” (See St. Augustine, Sermon 32)