I suggest that an argument which begins with P and ends with something logically equivalent to P cannot be usefully described as “Suppose P would be true? Then P would be true!” This makes OW’s hypothetical legitimate.
The argument has to go some distance. OrphanWilde is simply writing his hypothesis into his conclusion.
His hypothetical is “suppose atheism doesn’t win”. His conclusion is not “then atheism doesn’t win”, so he’s not writing his hypothesis into his conclusion. Rather, his conclusion is “then rationality doesn’t mean what one of your other premises says it means”. That is not saying P and concluding P; it is saying P and concluding something logically equivalent to P.
The argument has to go some distance. OrphanWilde is simply writing his hypothesis into his conclusion.
His hypothetical is “suppose atheism doesn’t win”. His conclusion is not “then atheism doesn’t win”, so he’s not writing his hypothesis into his conclusion. Rather, his conclusion is “then rationality doesn’t mean what one of your other premises says it means”. That is not saying P and concluding P; it is saying P and concluding something logically equivalent to P.