Then consider why Lewis believes that divorce (etc.) is harmful. It's arbitrary--if his religion had said something else, he'd have believed something else.

This is a universal argument. "Given a different history, you would have believed something else, therefore your actual belief is groundless." You can apply it to anyone, saying anything; which is to say, that it carries no force ever.

I suppose there's another possibility: Lewis doesn't want his religion to tell him

bong!!! But thank you for playing.

This is Bulverism, and not even Bulverism about a real characteristic, but about one you have just made up.