"God was teaching David lessons of trust.
Goliath trusted in his armor.
Saul had
his own kingly armor placed upon David.
But he would not consent to wear
it.
He did not mean that any
credit be given to the armor of Saul, for his trust was in the Lord God
of Israel. 
Choosing five smooth stones
out of the brook, he put them in his bag, and, with his sling in
his hand, drew near to the Philistine.
Those who bear the most solemn message ever given to our world must lay
off the pugilistic armor, and put on the armor of Christ’s
righteousness.
Amazement
spread along the lines of the two armies.
David's countenance
was ruddy with health, and his well-knit form, unprotected
by armor, was displayed to advantage; yet between its youthful
outline and the massive proportions of the Philistine, there was
a marked contrast.
Goliath was filled with amazement and anger. "Am I a dog," he exclaimed, "that thou comest to me with staves?" He cried in derision, "Come to me, and I will give thy flesh unto the fowls of the air, and to the beasts of the field."
They had been confident that
David would be slain; but when the 
stone went whizzing through the air,
straight to the mark, they saw the mighty warrior tremble, and reach
forth his hands, as if he were struck with sudden blindness.
The giant
reeled, and staggered, and fell prostrate to the ground. David did not
wait an instant. He knew not that life was extinct. He sprang upon the
prostrate form of the Philistine, and with both hands he laid hold of
Goliath's heavy sword.
A moment before, the giant had boasted that with it he
would sever the youth's head from his shoulders and give his
body to the fowls of the air. Now it was lifted in the air.
David did not weaken before the champion of the Philistines.
Stepping forward, he said to his antagonist: 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 carcasses 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." E.G.W.