To say that authority, whether secular or religious, supplies no ground for morality is not to deny the obvious fact that it supplies a sanction.
While moral rules may be propounded by authority the fact that these were so propounded would not validate them.
The traditional disputes of philosophers are, for the most part, as unwarranted as they are unfruitful.
There never comes a point where a theory can be said to be true. The most that one can claim for any theory is that it has shared the successes of all its rivals and that it has passed at least one test which they have failed.