In the context of 1 Samuel, nothing legitimizes David’s reign more than a confession from the lips of Saul himself that David would and had to follow him as king.1 Though he would not support the Lord’s decision, Saul seems to have come to a point where he recognized that his kingdom would be given to another (see 1 Samuel 13:14).2
20 And now, behold, I know that you shall surely be king, and that the kingdom of Israel shall be established in your hand.