John Gill Commentary 2 Samuel 15:35

"And hast thou not there with thee Zadok and Abiathar the priests? therefore it shall be, that what thing soever thou shalt hear out of the king`s house, thou shalt tell it to Zadok and Abiathar the priests." — 2 Samuel 15:35 (ASV)

And [have you] not there with you Zadok and Abiathar the priests ? &c.] To assist in forming schemes directly opposite to Ahithophel's, or to whom he could communicate the secrets of Absalom's court:

therefore it shall be [that] whatever you shall hear out of the king's house ; Absalom's, who was now in possession of the house and palace of David:

you shall tell [it] to Zadok and Abiathar the priests ; to whom he might have recourse without suspicion, pretending he had business with them as priests, on religious accounts, to offer sacrifices for him, &c.