David and Goliath…
The Philistine also said to David, “Come to me, and I will give your flesh to the birds of the sky and the beasts of the field.” (1 Sam 17:44)
Then David said to the Philistine, “You come to me with a sword, a spear, and a javelin, but I come to you in the name of the LORD of hosts, the God of the armies of Israel, whom you have taunted. (v.45)
“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, (v.46)
As David also stated, the battle is the Lord’s. (v.47) A smooth stone was sufficient to topple Goliath, nemesis of Israel.
But…how was it that David was so confident? Charles Spurgeon attributes this to all that time spent watching his father’s sheep. For, in those days and nights, David did far more than simply tend sheep. According to Spurgeon,
“When David was young in years, he was old in experience because he had watched the hand of the Lord in its dealings with him. He had not been an idler among the hills, but a worshipper, a worker, a student, a practical, living man of God. You must go and meet an experience if that experience is to bring you riches in both its hands. I mean that David’s experience was that God delivered him out of the jaw of the lion—but he, first, went and fought that lion by his own dauntless valor! He took the lamb out of his mouth and he laid hold upon his jaws and tore him in pieces. David went forth to meet that experience.
“And the bear that came to David—David did not sit still and watch the bear, let it come and roar, take its prey and then retreat as it liked—he struggled with that bear and he slew him! And thus he gained his experience by the active discharge of his duties as a shepherd. He did what he was called upon to do with holy daring and, in so doing, he learned the faithfulness of God. Many men have lions and bears, but no experience. Be alive and get something out of all that happens around you!” 
Spurgeon points out that David “had not been an idler among the hills,” but “a worshipper” and I have to ask myself, how often has time that could have been spent with Christ and His Word been given to other activities? Far too often, I have to admit.
Along with all he had to do, David worshiped. And thus he knew the Lord. That is why he was confident, because his faith was in the Lord. The bear and the lion had fallen; so would the giant who defied the army of the living God.
1. “The Lion And The Bear – Trophies Hung Up” – Sermon No. 1810 – Metropolitan Tabernacle Pulpit
Sermon Delivered on Thursday Evening, September 25, 1884, By C. H. Spurgeon, At Metropoolitan Tabernacle, Newington.