Albert Barnes Commentary 2 Kings 10:23

"And Jehu went, and Jehonadab the son of Rechab, into the house of Baal; and he said unto the worshippers of Baal, Search, and look that there be here with you none of the servants of Jehovah, but the worshippers of Baal only." — 2 Kings 10:23 (ASV)

The ancients usually regarded the presence of people from other religions as a profanation of their rites. In the case of the Greek mysteries, such an intrusion is said to have been punished by death. Consequently, Jehu could give these injunctions without arousing any suspicion.