Here's a thought experiment:
"Sin is the fault of humanity, but it had to be that way, and fortunately, we still have free will.
How do I know this to be true?
Well, for hundreds of millions of years, various animals and plants endured enormous levels of suffering before humanity existed. So it is difficult to claim that the direct actions of humanity are responsible for suffering in our world. However, one could say that suffering was predetermined for humanity. Essentially, it must then be our fault that the suffering of the world was predetermined for us. So everything's our fault (even if it's not), and we still have free will (even if we don't). Christianity makes complete sense."
Obviously, I'm being sarcastic here.
We can now determine that there was never a Paradise - that there was never an Eden - and that the "original sin" of humanity is merely our own existence in an indifferent universe. Sure, Christian tradition can try to pin the blame on a species which only arrived on Earth during the most recent instant of geologic time, but the growing evidence across nearly all disciplines of science can easily refute such a bold and unsupported assertion.
How can I prove that the only reason a god would permit evil is to bring about some other end? How can I know that this is not the only possible world that it is feasible for a god to create? I most likely cannot fully prove either of these things to you; what I may be able to demonstrate adequately is the incoherence of Christian dogma when its doctrines are contrasted with the harsh, vivid realities of our existence.