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 17:51)
In the Name + of Jesus. Amen. It seems like they always leave off that part of the David and Goliath story. It wasn't the stone that killed Goliath. David used Goliath's own sword to kill him and cut off his head!
That's what Jesus, the Son of David, does to the devil, our enemy. He steps forth and takes the enemy down, using his own weapon: death! The devil brought sin into this world and sin brought death. So Jesus uses death to take down the devil, by dying for our sins and so wiping out their power. By forgiving your sins, Jesus has cut the head off death so that it cannot be your enemy anymore. On the Last Day, Jesus will raise you up to live forever.
David, to prepare for fighting Goliath, took five smooth stones and was ready to launch them at the giant. So Jesus, pierced in hands and feet and side--five wounds!--fended off sin, death, devil and hell by his suffering and death.
Now, Jesus gives to us five smooth stones with which to defeat our enemies: His holy gifts. The Holy Spirit. Water. Word. Body. Blood. These gifts drive away your enemies and keep them from defeating you. In fact, with those gifts, you can tell off sin, death, devil and hell the way that David mocked Goliath and put him down. After all, those gifts of Jesus are your proof and promise that the Lord fights for you and therefore there is no enemy that is stronger than you.
David took down Goliath. Jesus takes down your big, bad enemies. There is nothing that can defeat you when your Good Shepherd and King fights for you, because He always wins. In the Name + of Jesus. Amen.
Down through the realm of darkness He strode in victory, And at the hour appointed He rose triumphantly. And now, to heav'n ascended, He sits upon the throne When He had ne'er departed, His Father's and His own. (Christ Is the World's Redeemer, LSB 539:3)