There may not be an objective right and wrong, but I believe there is an objective reality. Either a god or gods do exist, or not.
Since my philosophy says that, in the absence of clear evidence for its existence, God does not exist, I don’t believe in God’s existence. I believe that is the objective truth, although I could be wrong.
If it is true, there is no basis for a god-given morality. That follows entirely logically from the premise. My beliefs are entirely self-consistent, even if they aren’t consistent sometimes with what I feel, or would like to believe.
Now, you might say, if there is no objective morality, why is it important to me that I try to believe only in things that true? Well, it just is. It’s subjective. Regardless of what you, I, or anyone else believes, there is an objective reality that belief doesn’t affect. I can’t prove it, but it seems like a reasonable starting point. After all, if we don’t assume an objective reality, there is pretty much no underpinning for any system of beliefs.
But how do you know? Your belief seems to hang on subjective truth.