What does 1 Samuel 17:47 mean?
David responded to the challenge of the Philistine champion Goliath (1 Samuel 17:4–10). He will represent Israel in champion warfare against the giant (1 Samuel 17:32). Goliath is unimpressed and insulted by the youth in shepherd's clothes (1 Samuel 17:40, 42–44). But David is confident that God will bring the victory (1 Samuel 17:40). David comes not as a seasoned warrior, but as a faithful believer in the name of the Lord. The Philistine is no match (1 Samuel 17:45). David declares that he will strike Goliath down and decapitate him by that power (1 Samuel 17:46). Two reasons are given for why God will grant this victory. The first is so that the entire earth will know there is a God in Israel (1 Samuel 17:46).The second reason is so that everyone gathered on the field of battle will know that battles are not won by military tools, or even by men. God does not need the human advantages of better weapons or more soldiers or heavy armor. The Lord has all the power He needs to give victory to His people simply because He is the Lord. As the Lord later told Zerubbabel through the prophet Zechariah: "Not by might, nor by power, but by my Spirit, says the Lord of hosts" (Zechariah 4:6).
David understood his impossible-seeming victory over Goliath would be used by the Lord as evidence to both His enemies and His people; it would show that the Lord was the all-powerful God of Israel who could be trusted to deliver those who belong to Him.