The validity of external morality

I understand that you claim that there is no morality without G-d. Why not say that morality itself is found in a spiritual book external to the world where the rules of morality are “written,” and the claim: “x is moral” is tested by the conformity of the sentence to what is written in that book, and to discover what is written in the book, one can look within the mind. Then G-d is not needed.