| Matthew Henry's Concise Commentary 28:1-6 David could not refuse Achish without danger. If he promised assistance, and then stood neuter, or went over to the Israelites, he would behave with ingratitude and treachery. If he fought against Israel, he would sin greatly. It seemed impossible that he should get out of this difficulty with a clear conscience; but his evasive answer, intended to gain time, was not consistent with the character of an Israelite indeed. Troubles are terrors to the children of disobedience. In his distress, Saul inquired of the Lord. He did not seek in faith, but with a double, unstable mind. Saul had put the law in force against those that had familiar spirits, Ex 22:18. Many seem zealous against, sin, when they are any way hurt by it, who have no concern for the glory of God, nor any dislike of sin as sin. Many seem enemies to sin in others, while they indulge it in themselves. Saul will drive the devil out of his kingdom, yet harbours him in his heart by envy and malice. How foolish to consult those whom, according to God's law, he had endeavoured to root out!
Pulpit CommentaryVerse 2. - Surely thou shalt know. Hebrew, "Therefore thou shalt know," i.e. if the case be so, thou shalt know, etc. The rendering of the A.V. makes David repeat the words of Achish, which literally are, "knowing thou shalt know," the Hebrew way of making a strong affirmation. David's reply is really ambiguous, but is understood by Achish as a boastful assent, and he thereupon promises, Therefore will I make thee keeper of mine head, i.e. captain of my bodyguard, forever. Therefore is exactly the same word as that used by David, and has just the same meaning, namely, "If the case be so, if thou provest thy valour, then I, etc. SAUL AND THE WITCH OF ENDOR (vers. 3-25).
Gill's Exposition of the Entire BibleAnd David said to Achish, surely thou shalt know what thy servant can do,.... By which he would have Achish understand, and so he did, that he would exert himself in favour of the Philistines, and against Israel, and do great and brave things, of which Achish would be, made sensible, through his heroic courage and valour; though he meant rather what he could and should do for Israel against the Philistines, if he had an opportunity: but it seems best of all to consider David as quite undetermined, and at a loss what to do, hoping that God in his providence would extricate him out of this difficulty, and direct him what he should do, which then Achish would know; and accordingly he was delivered out of it: and Achish said to David: putting confidence in him on account of his answer, and believing he was hearty in engaging in the war with the Philistines against Israel: therefore will I make thee keeper of mine head for ever; the captain of his bodyguard, which post he should hold for life; or he proposed to put him into this post, that he might be with him, near his person, and under his eye, that he might observe how he behaved himself; which may show some suspicion of him.
Jamieson-Fausset-Brown Bible Commentary2. Surely thou shalt know what thy servant can do—This answer, while it seemed to express an apparent cheerfulness in agreeing to the proposal, contained a studied ambiguity—a wary and politic generality. Therefore will I make thee keeper of mine head for ever—or, "my life"; that is, "captain of my bodyguard," an office of great trust and high honor.
1 Samuel 28:2 Parallel Commentaries 1 Samuel 28:2 NIV
1 Samuel 28:2 NLT
1 Samuel 28:2 ESV
1 Samuel 28:2 NASB
1 Samuel 28:2 KJV Bible Hub: Online Parallel Bible | 
