Commentary of Charles Spurgeon, Adam Clarke & Matthew Henry
And he took his staff in his hand, and chose him five smooth stones out
of the brook, and put them in a shepherd's bag which he had, even in a
scrip; and his sling was in his hand: and he drew near to the Philistine. 
And the Philistine came on and drew near unto David; and the man that bare the shield went before him.
And when the Philistine looked about, and saw David, he disdained him: for he was but a youth, and ruddy, and of a fair countenance.
And the Philistine said unto David, Am I a dog, that thou comest to me with staves? And the Philistine cursed David by his gods.
And the Philistine said to David, Come to me, and I will give thy flesh
unto the fowls of the air, and to the beasts of the field.
Then said David to the Philistine, Thou comest to me with a sword, and
with a spear, and with a shield: but I come to thee in the name of the
LORD of hosts, the God of the armies of Israel, whom thou hast defied. This day will the LORD deliver thee into mine hand; and I will smite
thee, and take thine head from thee; and I will give the carcases of the
host of the Philistines this day unto the fowls of the air, and to the
wild beasts of the earth; that all the earth may know that there is a
God in Israel. And all this assembly shall know that the LORD saveth not with sword and spear: for the battle is the LORD'S, and he will give you into our hands.
Vs.40-47
Five smooth stones —
1. Had they been rough or angular,
they would not have passed easily through the air, and their asperities
would, in the course of their passage, have given them a false direction.
2. Had they not been smooth, they could not have been readily despatched from the sling.
Cursed David by his gods. — Prayed his gods to curse him.
This day will the Lord deliver thee into mine hand — This was a direct and circumstantial prophecy of what did take place.
1. How very proud Goliath was,
(1.) With what scorn he looked upon his adversary. He looked about, expecting to meet some tall strong man, but, when he
saw what a mean figure he made with whom he was to engage, he disdained
him.
(2.) With what confidence he presumed upon his success. He cursed David
by his gods, imprecating the impotent vengeance of his idols against
him, thinking these fire-balls thrown about him would secure his
success: and therefore, in confidence of that, he darts his grimaces, as
if threatening words would kill: Come to me, and I will give thy flesh to the fowls of the air.
2. How very pious David was. His speech savors nothing of ostentation, but God is all in all in it.
(1.) He derives his authority from God: I come to thee by warrant and commission from heaven, in the name of the Lord, who has called me to and anointed me for this undertaking.
(2.) He depends for success upon God. David speaks with as much assurance as Goliath had done, but upon better ground; it is his faith that says, This day will the Lord deliver thee into my hand,
and not only thy carcase, but the carcases of the host of the
Philistines, shall be given to the birds and beasts of prey.
As for those stones out of the brook, David did not pick them up at
hazard; he carefully chose them, selecting smooth stones that would
exactly fit in his sling the kind of stone he thought best fitted for
his purpose. Nor did he trust in his sling. He tells us he trusted in
God, but he went to work with his sling as if he felt the responsibility
to be his own. To miss the mark would prove his own clumsiness: to
compass his aim would be of God's enabling.
