1 Samuel 24:20 - Ellicott's Commentary On The Whole Bible

And now, behold, I know well that thou shalt surely be king. — Clericus (in Lange) says: “From this great magnanimity of David, Saul concluded that a man who was much superior in soul to kings could not but reign.” This is a good comment, and doubtless expresses something of what was in Saul’s mind on this occasion; but more must have been behind to have induced the king to make such a speech to David. Never had he for one moment forgotten his old friend’s words — the words of Samuel, whom he too well knew was the prophet of the Most High — when he with all solemnity announced to him, as a message from heaven, that the Lord had rent the kingdom from him, and had given it to a neighbour that was better than he (1 Samuel 15:21). Since that awful denunciation, the unhappy Saul was only too sensible that the blessing of Jehovah of Hosts no longer rested on his head, no longer blessed his going out and coming in, while the strange, bright career of the son of Jesse seemed to point him out as the neighbour on whom the choice of God had fallen. Rumours, too, of a mysterious anointing must have long ere this reached Saul; this, joined to the passionate advocacy of Jonathan, and the quiet, steady friendship of Samuel, no doubt convinced King Saul that in the son of Jesse he saw Israel’s future monarch. Strong, therefore, in this conviction, and for the time humiliated and grieved at the sorry part he had been playing in this restless persecution of one destined to fill so great a position, the king positively entreats the outlaw to swear to him the strange promise contained in the next (21st) verse.