Impossible to Keep the Law?

It is often stated that it is impossible for us to Keep the Law - or at least it must be impossible without first receiving the Holy Spirit. Studying the OT closely, I keep finding scriptures that assert otherwise (like Moses’ speech in Deuteronomy 30). King Saul not only continues to sin after receiving the HS, but sins so greatly that God revokes his Spirit from Saul! (So much for once saved always saved). And even King David - who is otherwise treated as the Gold Standard for OT Kings - commits sins that the Law demands death for after receiving the HS. After completing another read-through of 2 Kings, I found what appears to be the perfect counter example: Josiah. After learning of the Book of the Law, Josiah re-instates the Covenant and carries out his duties to rid the land of the worship of foreign gods and other sinful practices. No fault is listed against him, and in the final analysis he is raised up even over King David: 2 Kings 23:25 > Before [Josiah] there was no king like him who turned to the Lord with all his heart, all his soul, and all his might, in conformity to all the Law of Moses; nor did any like him arise after him. It is worth noting that not only did Josiah perfectly follow the Law, but he also did so without having been blessed with the HS as his predecessors Saul and David had been. How do those that maintain that only Christ and/or only those that have the HS can keep the Law reconcile that with King Josiah?