1 Samuel 17:51
Therefore David ran, and stood upon the Philistine, and took his sword, and drew it out of the sheath thereof, and slew him, and cut off his head therewith. And when the Philistines saw their champion was dead, they fled. Therefore David ran, and stood upon the Philistine, and took his sword, and drew it out of the sheath thereof, and slew him, and cut off his head therewith. And when the Philistines saw their champion was dead, they fled.
This verse belongs to the account of God saving Israel from the Philistines through David, who was sent on a divinely directed mission and whom God had raised up to rule. David was moved by zeal to defend the honor of God and His people to face Goliath. In doing so he answered the champion's defiance of Israel and its God at a time when Saul and the army were afraid and unable to act.
After the stone had struck Goliath down, David completed the victory the Lord had given him by drawing the fallen Philistine's own sword and cutting off his head with it. That sword was the weapon Goliath had boasted in. When the Philistines saw their champion was dead they were confused and fled in every direction, which turned the battle against them. Israel's camp rejoiced, and Israel pursued them.
The victory cost the Philistines their champion. David's valor was remembered by them, so that years later people still identified him as the man who had slain the champion. That fame endangered him when he sought refuge with Achish.
The chapter teaches that God's servants must not take Goliath's part by defying and provoking discussion but must leave defiance to opposers of truth. Going forth confident and boastful like Goliath, magnifying self instead of hiding behind Jesus, and trusting in prepared arguments as Saul wanted David to trust in armor rather than humbly trusting in the God of Israel and making Him their strength, throws them off balance. That course makes discussions fail, and injures their own souls and the souls of others.