Rawls: What does justice require? Let's imagine a bunch of people have to choose principles of justice to government their society, a society they will be born into and which they can leave only through death. I hereby stipulate that these people want to ensure they have happy life. I hereby stipulate that they are aware that people have diverse conceptions of what the good and happy life is. I also hereby stipulate that they won't know which conception of the good life they themselves will have, and further, that they have no idea about the probability distribution of such conceptions. It could turn out 99% percent of people have some conception, or 0, or 1 in 20,000,000, or anything else. So, my hypothetical reasoners will choose a tolerant principle which allows people to live out a very wide range of possible lifestyles. Bingo!
Radical Christian conservative communist: But what if God actually wants us all to live in small-scale Christian anarchist communes with traditional marriage roles and very limited lifestyles?
Rawls: My theory rules it out!
RCCC: Yeah, but who cares? Why would the fact that your theory rules that out tell me that my view is mistaken?
Rawls: [Repeats his theory again. That's how he rolls.]
RCCC: Right, but you're missing the point. Your decision-procedure is more or less designed to get a specific kind of output and to rule out my view. But that doesn't tell me that your view is right and mine wrong. Do you have an independent argument that tells me why your particular thought experiment is supposed to reveal the correct theory of morality or justice?
I mean, you're saying that a bunch of selfish people who didn't know what their own private opinion of the good life is would pick an open liberal society. Yeah, I can see how selfish people would do that. But what does that have to do with morality? Why is ethics what selfish but ignorant people would choose if they didn't know who they are?
God: Hey, everyone, I just want to clarify that the RCCC is correct. That's what ethics requires.
Rawls: Sorry, my theory rules it out.
Now, obviously, if God appeared and said that, Rawls would have to concede. I think even Rawls would not be willing to call God unreasonable. And of course we don't have good reason to think God has actually said this. But my point is that Rawls's theory simply rules out the possibility that the RCCC is right without what would count as a non-question-begging argument against it.