Albert Barnes Commentary 2 Samuel 15:12

"And Absalom sent for Ahithophel the Gilonite, David`s counsellor, from his city, even from Giloh, while he was offering the sacrifices. And the conspiracy was strong; for the people increased continually with Absalom." — 2 Samuel 15:12 (ASV)

Ahithophel - It has been supposed with great probability that Ahithophel was estranged from David out of personal resentment for his conduct in the matter of Bathsheba and Uriah (see 2 Samuel 11:3).

While he offered sacrifices - Rather, Absalom sent for Ahithophel to be present when he offered the sacrifices; the intention was that all who partook of the sacrifice would be bound together to prosecute the enterprise. Absalom, too, would take advantage of the excitement of the great feast to inflame the zeal of the guests and pledge them irrevocably to his cause.