What does 1 Samuel 17:15 mean?
The Philistines and Israelites are at war (1 Samuel 17:1–3). A Philistine champion of enormous stature with impressive armor has challenged the Israelites to champion warfare (1 Samuel 17:4–10). The Israelites respond in fear (1 Samuel 17:11). With the tension built, the text re-introduces David. If reading the entire scroll of Samuel, the reader is aware of David already (1 Samuel 16). The reintroduction makes this particular battle narrative able to stand on its own—able to be read without needing to include the entire last chapter.David is the youngest son of an elderly man from Bethlehem named Jesse. David's oldest three brothers are at the battle; his other four brothers are not mentioned (1 Samuel 17:12–14). David goes between serving Saul and caring for his father's flocks. At this time, David is not considered a full-grown adult. However, he is no child (1 Samuel 17:34–36).
The prior chapter provides helpful context. Though Saul maintained his earthly reign, God had rejected Saul as king (1 Samuel 13:13–14; 15:22–29) and chosen a new king. God sent the prophet Samuel to Bethlehem to anoint that king from among Jesse's sons (1 Samuel 16:1). The first seven sons were rejected (1 Samuel 16:6–10), so Samuel asked Jesse if there were any more. David, the youngest, was out caring for the sheep and apparently not invited to the sacrifice and the feast Samuel had come to conduct (1 Samuel 16:11). But he was sent for and anointed (1 Samuel 16:12). The Spirit of the Lord rushed on David and remained with him (1 Samuel 16:13).
In the meantime, the Spirit left King Saul. A harmful spirit tormented the king, and his advisors suggested he employ a musician to bring relief. Knowing David to be a skilled musician, a man of character, and someone with whom the Lord's presence was evident, a servant suggested him. The king quickly loved and trusted David, and the tormenting spirit left when David played (1 Samuel 16:14–23). Saul asked for David to remain in his service, but evidently David still went back and forth between regular service to Saul and his father's house. It's possible that this incident with Goliath is what made Saul promote David to carry his armor (1 Samuel 16:21).