What does 1 Samuel 17:13 mean?
During a narrative about Philistines challenging Israel to champion warfare, David and his family are re-introduced to the reader (1 Samuel 16). This battle is crucial in David's role as king. The imagery also alludes to the future Messiah, who is from David's line (Genesis 3:15; 2 Samuel 7:12–17; Revelation 20:2, 7–15). It makes sense that this narrative be able to stand alone; thus, David would need to be introduced. This re-introduction also reminds readers of what occurred previously. It's also possible that the earlier description of David in Saul's service (1 Samuel 16:21) includes what happened with Goliath, saving details for later.The prophet Samuel was sent to Jesse's home to anoint the new king of Israel. Though Samuel thought the eldest would be the anointed, he was not God's chosen. Neither were the other six brothers. Instead, it was the youngest son, who was out in the fields with the sheep, who was God's chosen (1 Samuel 16:1–13).
Here we see that Jesse's three oldest sons are at the battle. It would have been expected that men of fighting age would join the army of Israel during times of war. This is likely why Jesse is described as being older in the previous verse, to explain why he stayed home (1 Samuel 17:12).
Eliab, Abinadab, and Shammah do not answer Goliath's threat. Neither does the sitting king, Saul. Confident in God, David does (1 Samuel 17:31–32). This account demonstrates that David is, indeed, anointed by God and willing to act in faith.