There shall not any man be able to stand before you all the days of your life. As I was with Moses, so I will be with you; I will not fail you, nor forsake you.
Josiah's reforms led to a popular observance of the sabbath, and of the Passover, (2 Kings 23:22).  How did they know how to keep such laws?  Where else but in the Pentateuch could the instructions be found?