1 Samuel 28:17,18 - Joseph Benson’s Commentary on the Old and New Testaments

The Lord hath rent the kingdom out of thy hand, &c. Here the prophet foretels that Saul should that day be stripped of the kingdom, and that it should be given to David. Then follows what nothing but infinite, unerring prescience could predict; an exact, minute, precise account of all the circumstances of the then depending event! Because thou obeyedst not the voice of the Lord Saul's sin in killing the Lord's priests, and in seeking to kill David, is not here mentioned, because the decree of taking the kingdom from him was passed before those sins were committed. Delaney asks here, “Would an impostor” (for such this apparition must have been, if it were not Samuel) “have been so very zealous for a strict observance of the law and commands of God; and so rigid in pronouncing divine vengeance upon the violation of them; and in the depth of his cunning have limited that vengeance to time, place, and person; and all this at no greater distance than the next day? These suppositions are too wild to be seriously confuted; they are the very reverse of what should and would have been done on such an occasion, had imposture interfered in it. Every one knows the business of impostors is to flatter, to delude, to deceive, to answer doubtfully; to promise good and put off the evil; it was this woman's business in a particular manner to act thus. Had she promised Saul victory, and the success had answered, she was sure of considerable advantage. He, who could have no benefit from priests or from prophets, would, doubtless, have had her in high honour, and with good reason. If he died in the battle, all was safe; and even if he escaped and was worsted, what she said would at least have been taken for an indication of good wishes to the king and to his people; and so would be more likely to escape any after inquiry. Whereas, if she prognosticated evil to the royal race, she was sure of destruction, if the event did not at once justify and save her.”