Even then, though, Jehu wasn't careful to walk in God's ways and honor the God of Israel from an undivided heart. He didn't turn back from the sins of Jeroboam son of Nebat, who led Israel into a life of sin. 2 Kings 10:31 (MSG)
There is a difference between believing God and following God. Acknowledging is not the same as submitting. Walking a personal path that parallel's God's for a stretch is not the same as walking God's path.
Jehu was not a God follower. He acknowledged God just as far as it would serve his own purpose. He was on the throne, the extensive family of Ahab was wiped out, the Baal cult that had been such a powerful adversary had been eliminated to ensure the security of his reign. God had served Jehu well. In the end, the opposite was not true.
Is God serving my purpose(s), or am I serving God's purpose(s)?