Question: "Why does Saul ask who David is, when he had met him earlier (1 Samuel 17:55)?"
Answer: First Samuel 17 relates the famous battle between David and Goliath. Young David had heard and seen the giant Philistine taunting the armies of Israel, and David said he would volunteer to fight him (verse 32). Word of David’s offer reached King Saul, who summoned David, interviewed him, and sent him out to fight Goliath (verses 31–37).
As David went out to face the giant, Saul looked on. The king then asked Abner, the commander of the army, “Whose son is that young man?” (1 Samuel 17:55). This question has caused some puzzlement, since in the previous chapter, David had been employed to play the lyre for Saul. The king knew David’s name and had been told who his father was (1 Samuel 16:14–23).
There are several possible explanations for why Saul asks about David in chapter 17 after he had been introduced to him in chapter 16. As a preface, we should note that Saul was not inquiring of the identity of David per se in chapter 17; rather, he was asking who David’s father was (verses 55, 56, and 58). We assume he knew David’s name. David’s answer, in verse 58, is, “I am the son of your servant Jesse of Bethlehem.”
One explanation for why King Saul asked the identity of David’s father is simply that he had forgotten whose son David was. Saul had been told about Jesse (1 Samuel 16:18), and he had even had correspondence with him (verses 19–22), but it’s not unreasonable to think that the name had slipped Saul’s mind. We assume that kings have a lot of information to keep track of, and we understand if Saul did not regularly review the names of all the people in his realm.
Saul needed to know the name of David’s father to deliver the prize promised to the one who defeated Goliath. In 1 Samuel 17:25, Saul had promised great wealth and the king’s daughter in marriage to the man who killed the giant. Saul had also promised the family of the victor tax exemption. In order for Jesse to receive this benefit, Saul had to confirm that’s who David was.
Another possibility is that Saul’s question was meant to inquire of David’s background in general, rather than the specifics of his family. Where does this boy come from? From what tribe is he? What clan?
Another possible explanation for why Saul asked the identity of David’s father is that the events of 1 Samuel 17 occurred chronologically prior to those of chapter 16. In this case, David’s stepping forward to fight Goliath was the first time Saul had met David.
In any event, the killing of Goliath proved that David was a truly obedient servant of God who was concerned with the interests of his Heavenly Father and his Father’s people (cf. John 8:29). Jesus, the Son of David, likewise stepped forward in the power of God to slay the giants who kept us cowering in fear (see Colossians 2:15 and Revelation 5:5).