“In the One Year Bible reading from 1st Samuel today, David shows up at the battle with the Philistines to bring food to his brothers and ends up killing Goliath. In previous chapters David played and sang to Saul when he was ill. So why did Saul not know David at this battle or is the book out of order chronologically? Also, David had been anointed by Samuel as king before this.”
Because the two mentions of David playing music for Saul are so close together in the same book, but in different settings, I do not think this is a mix up in the chronological order of the story (16:23 and 19:9). Perhaps the reason that Saul did not recognize David when he came to fight against Goliath is because as 17:15 tells us, David was only a part time servant. He would go back and forth from Saul’s court to Bethlehem to help his father with the sheep.
We are told in 1 Kings of King Solomon, “But Solomon did not make slaves of any of the Israelites; they were his fighting men, his government officials, his officers, his captains, and the commanders of his chariots and charioteers.” This gives us a hint of how many leaders the King oversaw without even mentioning wives, children and a plethora of servants. Given how many people were in a king’s court, it is not too much to imagine that he would not remember a part time servant. Add to this that Saul was under the stress of battle and he could be excused for a lapse in memory. Also we do not know how much time transpired between the time that David first became Saul’s servant and when he returned from helping his father to fight against Goliath. Even a few years in a young boys life can make a considerable difference in appearance as they become men.
As far as the anointing of David, that was a private affair just among his brothers (1 Samuel 16:13). Saul would not have known about it at that point. David’s public anointing as King of Judah would not come until after Saul and Jonathan’s death (2 Samuel 2).