In 1 Samuel 17 there is again mention of a Philistine advance against the Israelites. King Saul gathers his army to stop the raid. The armies of the Philistines and the Israelites are arrayed on either side of the valley of Elah; the ‘Valley of the Terebinths,’ which is about 15 miles (25 km) west of Bethlehem.
Because of his disobedience in the earlier battle against the Amalekites (1 Samuel 15), King Saul has forfeited the favor of God. The prophet Samuel must then secretly anoint young David, the son of Jesse, as king. 1 Samuel 16.
David's victory over Goliath draws the people's attention to David, and Saul's kingship again loses authority.
Goliath, a Philistine camp fighter about ten feet tall and wearing impressive armor, challenges the Israelites to send a man to fight him. Both men then fight alternately for their armies. However, Saul and the Israelites fear Goliath.
When David enters the Israelite camp, he says, "Who is this uncircumcised Philistine, that he should dare to mock the ranks of the living God?", verse 26. These words can be understood against the background of the Deuteronomy commission in chapter 7, where Israel is commanded by God to 'banish' or exterminate the Canaanite peoples. These peoples are being driven out of the land of Canaan as a punishment for their sins. God promised the land Canaan to Israel. Israel, however, is no more than an instrument in God's hand, for He Himself leads the battle. As the battle against Jericho was led by the Commander of the Lord's army, so David knows that "the battle is of the Lord" even now. See verse 47. That gives him tremendous courage of faith.
He meets Goliath "in the name of the LORD of hosts," and kills Goliath.
God the LORD is the true King of Israel. David will later be “viceroy” under God.
The battle against Goliath, who wears scaly armor and is mortally wounded in the head, is reminiscent of Genesis 3:15, where it is promised that Eve's descendant would crush the (scaly) serpent’s head. That promise is fulfilled in Jesus, the Son of David, who crushed the head of Satan, the "old serpent" (Revelation 12:9) on Calvary, though His outward form was feeble.
David's fight against Goliath foreshadowed this. It was a spiritual battle for the honour of God's Name.
In the spiritual battle that still has to be waged, Christians can only overcome in God's power.
Philippians 4 vers 13: ‘I can do all things through Him who strengthens me.’
John 15 verse 5: ‘ for apart from Me you can do nothing.’