An argument defeating the "Free Will defense" of the problem of evil.
Epistemological justification for adopting this form of argument:
it is a valid deductive proof (as far as I can tell, although I admittedly skipped a couple steps - such as from 4 to 5, because they seemed trivial), so the conclusion is necessarily true if the premises are true. Further, the truth of the premises is more plausible than the converse - so it's reasonable to believe them (again, as far as I can tell).
I'm not claiming the argument has dialectical efficacy - i.e. that it can persuade a Christian. Rather, it is reasoning that a person should consider who is having doubts about God as a result of considering the problem of evil. The free-will defense is often presented as a defeater of the argument from evil, and I'm presenting this as a defeater of THAT defeater.
I agree it doesn't defeat all possible theological arguments, just the one I alluded to. I'll add that the "free will defense" only addresses the evil performed by free-willed individuals; there are other evils in the world - but that's beyond the scope of this discussion.
There is a tension between God's omnipotence and his inability to create free-willed beings that do not sin. It's the crux of my argument, and it also has bearing on the atonement. This also is beyond the scope of the present discussion.