1 Samuel 17:55

Whose son is this youth? It may, at first sight, appear strange that Saul should be represented here as not knowing who David was, when we have a relation in the foregoing chapter of his sending for him to court, being highly pleased with his behaviour, and much delighted with his music, making him his armour-bearer, and sending to his father Jesse to ask his leave for his continuance at court. But it may be observed that Saul, in this place, does not express an entire ignorance of David, but only inquires whose son he was A question of the more consequence to him, as he had promised his daughter in marriage to the conqueror of Goliath. Either Saul had never before made any inquiry about his parentage, or both he and Abner had forgotten whence he was. And this might very easily happen to a king and a general of an army, who daily see and have to do with so many different faces, and who pay so little regard to things of this sort. Nay, if Saul had entirely forgotten David, it would not have been strange, considering that he had been but little with him, had some time ago been dismissed from the court, and was returned home, where he had remained at least a year or two, during which time Saul had not seen him. Besides, the distemper of Saul's mind might make him forgetful, and David might now be much changed, both in his countenance and in his habit. Abner said, I cannot tell Abner's employment was generally in the camp, when David was at the court; and when Abner was there he probably took little notice of a youth so much inferior to him as David was.

“We may learn from the whole of this pleasing chapter,” says Dr. Dodd, “how ready God is to help those who trust in him; for whose defence and protection he makes use of means apparently the most weak, to humble the pride of the wicked, and to destroy the powers which seem most formidable. Some writers have considered this destruction of Goliath by David as a type of the victory of Jesus Christ, in his state of weakness and humiliation, over the strong and gigantic powers of hell and the grave.”