The only steelman I can come up with here starts by assuming that he considers Authority a terminal or root moral value. Then he could correctly argue that atheism leaves no basis for believing in Authority as a fundamental value. Neither does theism, unless you specifically work that in - but let's ignore this. Certainly I would expect a greater tendency to believe in this value among theists.
The main problem lies in the fact that I don't need to believe in Authority to oppose murder. He picked an example that has nothing to do with it, precisely so that atheists would agree he was talking about something bad. If he chose some 'sin' that only seems bad if we treat Authority as fundamental, then a lot of us would proudly say we don't see a problem with it. And he would more obviously be committing a logical fallacy, in this case begging the question by assuming his disputed morality is better than ours.
I think it is a level subtler than that. Value is downstream from utility - we consider something good because it is good for something. Most values are instrumental. Terminal values are a bit hanging in the air. The theist solution is to call terminal values simply instrumental values for god's purposes and call it a day. I.e. humans practically being gods property or tools. That way all values are instrumental, all goods are good for somethings and it is coherent.
The interesting part here is that if feels seductively intelligent. After all most people ...
Link to Blog Post: "Extremism in Thought Experiments is No Vice"
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