I am using examples related to racism because I think it's something most of us would agree is morally wrong, and our conviction on this is going to be pretty tough to shake. Or I think so. I'm in no way implying that Calvinism is implicity racist.
I will divide the claims of Calvinism into two propositions.
1) There exists a supremely powerful being who has predestined some to heaven and others to hell.
2) That being is morally justified in so doing.
2 seems false to me, but if I cam to think 1 were true, I would certainly have to entertain some questions about 2. That's where the humility comes in.
However, what I won't accept as an argument for 2 boils down to argument from might to right. So the writing in the sky wouldn't persuade me of the rightness of these actions, because that would only show might and would not be a moral argument.