As far as I can tell, "something that you would only do if God cannot exist" refers to an empty set.

It may be, since I gave a declarative definition rather than a constructive one.

People aren't objecting to particular scientific opinions held by Chesterton, but to how Chesterton's standards lead one along bad paths in general. Some things have fences for no reason than bigotry.

I actually gave my own objection to the Fence thing in my own comment, namely that fences are often there simply because some powerful tyrant wanted them, and nobody else wants them.