"Then he took his staff in his hand; and he chose for himself five smooth stones from the brook, and put them in a shepherd's bag, in a pouch which he had, and his sling was in his hand...Therefore David ran and stood over the Philistine, took his sword and drew it out of its sheath and killed him, and cut off his head with it. And when the Philistines saw that their champion was dead, they fled" (1 Samuel 17:40,51).
The story of David and Goliath is an icon of Scripture. Even those in the world know what happened. When they want to illustrate a fight with a giant of some sort, they often refer to it as "David versus Goliath."
The Philistines had lined up against Israel's army and day after day, each morning and evening, their champion taunted God's people. Goliath's taunting was so daunting, every soldier in Israel trembled.
But the Philistines did something that wasn't too intelligent. They put all their eggs in one big basket: Goliath. They were so sure that no one could beat their nine-foot-ten giant, they made no provision for his defeat, and when he went down, they fell with him. When one shepherd boy took down their weapon of mass destruction, they were forced to flee for their lives. They didn't have a second string to their bow.
The world puts all of their eggs in one basket when it comes to their salvation. They champion self-righteousness and make no provision for its failure to justify them on Judgment Day. But on that terrible Day there will be nowhere to flee. Not even the rocks will hide them from their enemy—Eternal Justice. And it's because of that terror that we must do all we can to reach them with the glorious gospel.
Notice what it was that David had in his arsenal. He took in his hand his staff, five smooth stones, and, of course, his trusty sling. Without it, the stones would be useless.
God has given the Church the fivefold ministry listed in Ephesians 4:11: apostles, prophets, evangelists, pastors, and teachers. But notice that four of these are to serve within the Church, while the evangelist has a concern for those outside the Church—the unsaved. His passion and ministry is to equip the Church to reach the lost, and for that he carries, like Moses, a special rod of authority that moves the hand of God. On the day David met Goliath, it typified his God-given authority to take down the giant.
We have been given authority in Christ: "And Jesus came and spoke to them, saying, 'All authority has been given to Me in heaven and on earth. Go therefore and make disciples of all the nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all things that I have commanded you; and lo, I am with you always, even to the end of the age'" (Matthew 28:18–20).
In the gospel, we have been given one special smooth stone to defeat the enemy. But it does no good inside the Church; we must take the gospel in hand and place it in the sling of God's Law as Jesus did in Mark 10:17. It's the Law that gives the gospel its thrust. It's the Law that makes the gospel message make sense to the unsaved, leaving a deep impression on his mind. The Law is a tutor that brings him the necessary understanding (Galatians 3:24). It gives him light, and it's in his conversion that we get our victory and send the demons of darkness fleeing.
So when you face the daunting enemy, don't forget the authority you have in Christ. Don't forget the smooth stone of the gospel—because it's the power of God to salvation (Romans 1:16). And don't forget to put the gospel in the sling of God's Law, to give it the thrust it is divinely designed to have.