When Saul saw David go forth against the Philistine, he said to Abner, the captain of the host, Abner, whose son is this youth? Abner said, As your soul lives, O king, I can't tell.
But why did neither Saul or Abner recognize David.  Simply because it had been a long time since David and played the lyre for Saul, and the changes in David's appearance in the meanwhile make their recognition impossible.  As the splendid scholar Robert Jamieson stated it, "The growth of the beard and other changes in the now full-grown youth prevented the king from recognizing his former favorite minstrel."[21]      Thus, in one day's time, God set in motion the events that would eventually elevate David to the throne as Saul's successor.