The law books used for the great reformation of Israel's social and religious institution were found in the temple in ____ BC during the reign of _____.

The law books used for the great reformation of Israel's social and religious institution were found in the temple in 621 BC during the reign of Josiah. To explain further, Josiah was a king of Judah who ruled from 640-609 BC. In the course of repairing the temple in Jerusalem, the high priest Hilkiah discovered a book that was later identified as the Book of the Law, which contained the laws and commandments given by God to Moses. Josiah recognized the significance of the discovery and initiated a series of religious reforms based on the teachings in the book. This event is described in the Old Testament book of 2 Kings 22-23. Therefore, the correct answer is 621, Josiah.