My comment previously (see 1 Samuel 14) about Saul trusting in himself and his own ability to wage war is underscored in chapter 17. Remember that in chapter 13, Israel had no weapons. The only people with a sword were Saul and Jonathan. Though the Philistines were driven back to their own borders at the end of chapter fourteen, they have returned in seventeen, almost at Bethlehem itself.
David acts in the Spirit of Jonathan (compare 14:10,14 and 17:37, 45-46), and you can understand why they would become such close friends. Furthermore, David acts without sword or spear (17:47) and triumphs over Goliath with a mere sling and a rock. It reminds me of the woe in Isaiah: “Woe to those who go down to Egypt for help, who rely on horses, who trust in the multitude of their chariots and in the great strength of their horsemen, but do not look to the Holy One of Israel, or seek help from the Lord” (31:1). This is the real message of the chapter: there is nothing that can stop a person of God who acts in the strength of God to accomplish the will of God.