20 Then Absalom spoke to Ahithophel, "Are you ready to give counsel? What do we do next?" 21 Ahithophel told Absalom, "Go and sleep with your father's concubines, the ones he left to tend to the palace. Everyone will hear that you have openly disgraced your father, and the morale of everyone on your side will be strengthened." 22 So Absalom pitched a tent up on the roof in public view, and went in and slept with his father's concubines. 23 The counsel that Ahithophel gave in those days was treated as if God himself had spoken. That was the reputation of Ahithophel's counsel to David; it was the same with Absalom.
Matthew Henry's Commentary on 2 Samuel 16:20-23
Commentary on 2 Samuel 16:15-23
(Read 2 Samuel 16:15-23)
The wisest counsellors of that age were Ahithophel and Hushai: Absalom thinks himself sure of success, when he has both; on them he relies, and consults not the ark, though he had that with him. But miserable counsellors were they both. Hushai would never counsel him to do wisely. Ahithophel counselled him to do wickedly; and so did as effectually betray him, as he did, who was designedly false to him: for they that advise men to sin, certainly advise them to their hurt. After all, honesty is the best policy, and will be found so in the long run. Ahithophel gave wicked counsel to Absalom; to render himself so hateful to his father, that he would never be reconciled to him; this cursed policy was of the devil. How desperately wicked is the human heart!