To be all formal for a moment and reference an actual philosopher, the estimable Socrates, who is still a guiding light despite being filtered through the ruinous window that is Plato, offered us a sterling thought experiment.
Are events or actions good because god ordains it so, or does god sanction those events or actions it deems good?
If the divine nod is given to the already good this implies a source of moral authority independent of, and perhaps prior to, god. And we do not need god to be moral agents. Unpalatable to some (and this is a factor in truth?). However, if the good is good by divine fiat then morality is reduced to the whim of a tyrant – murder can be good if such is decreed.
Elegantly, appeals to why no god would sanction murder collapse back into the first implication – independent, and perhaps prior, sources of moral authority.
So far the Athenians got. Impressive, often-cited today, yet largely ignored. Why no further? Perhaps because of the most unpalatable thought at all. We are all the source of moral authority. We are the whimsical tyrants. And we are not comfortable with the responsibility. We should not wonder why we strive to pass this buck to the invisible and unaccountable. Yet until we accept it, as long as we refuse authority, we remain the slaves of good and evil, not their rulers and adjudicators.