(In which I get overly lyrical.)
One way to justify Divine Command Theory - the idea that a God can provide objective morality - is to say that "what is good is good because God commands it". I'm having trouble marshalling a coherent argument against the that idea, so until then, here's my intuitive problem with it: even if I were to receive convincing proof that God exists and has laid down certain commandments, that would not replace my inner sense of morality.
I recoil instinctively from the idea that the commandments of God cannot be morally assessed - like Job, I want to challenge what I think is unfair. If I'm wrong, I want good reasons. In fact, my problems with Divine Command Theory can be pretty well sketched out with constant reference to Job. When God speaks from the whirlwind and Job backs down, I don't see that as a triumph of morality over pride. I see that as a triumph of power over reason.
In Job, we see that like gravity on spacetime, power exerts a distorting influence on morality. Infinite power, like a black hole, can collapse everything around it to become the only relevant point in the moral landscape. But I reject this framework! Like the Stoics, I want ethics to be the one thing untouched by power. I want the courage to stand in Job's position
and tell God that despite the torture - and it is torture - I will continue to demand a reckoning. He can take everything from me, including my life, but the only person who can bend my ethics is me. If God reaches out to alter my ethical framework, I cease being me anymore - it is as if he has killed me, and so I win.