A few scattered thoughts on 1 Samuel 17.
Saul offers David his armor for battle, but David refuses (v. 39). After the battle, though, David accepts the armor of Jonathan, signifying David’s elevation to crown prince (18:1-4).
David kills Goliath with a rock, trusting in Yahweh the Rock. There is a cosmological and political dimension to this: David takes the stones from a brook (17:40), land from the water (cf. Day 3), and in killing the Philistine he restores the land to Israel, driving back the sea of Gentiles that had engulfed the land.
David’s stone hits Goliath in the “forehead.” Prior to this, the forehead has only been mentioned as the place where Aaron bears the gold “flower” of the high priest’s crown (Exodus 28:38). Wearing a stone embedded in his forehead, Goliath is an anti-priest.
The chapter ends with Saul’s strange inquiry about David’s father. This is prepared earlier in the chapter, since Saul promises benefits to the family of the man who defeats Goliath (v. 25).
But it also picks up on a father-son theme from earlier in the book. When Saul became king he met a group of prophets and, gripped by the Spirit, prophesied (1 Samuel 10). “Is Saul also among the prophet?” everyone asked, and then “And who is their father?” The point is that in being filled with the Spirit of the prophets, Saul’s parentage had changed; he was no longer son of Kish but one of the sons of the prophets, his father Samuel. Saul’s rebellions against Samuel are all violations of the fifth commandment.
Now David, having been anointed with oil and the Spirit, having defeated Goliath, is also questioned about his father. He is in transition from one father to another. Soon he will be son-in-law to the king; he will start wearing Prince Jonathan’s armor. He has become son of Saul, and that intensifies the tragedy of Saul’s murderous hostility toward him. As he tried to kill Jonathan earlier (ch. 14) so he will try to kill his new son.
Saul becomes a Pharaoh, but worse: He slaughters his own sons.