Only be strong and very courageous, to observe to do according to all the law, which Moses my servant commanded you: don't turn from it to the right hand or to the left, that you may have good success wherever you go.
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?