What if we discover that consciousness is no more real than luminiferous ether, and that both humans and machines are just patterns for processing and acting upon the environment and not fundamentally different.
I always wondered how people were capable of thinking that. I'm atheist myself, but I don't think consciousness can be explained away so easily. For one, we are observing something, there is something in that meat that is seeing the images our eyes process. It's one thing to doubt the consciousness of others as you can't directly observe it, it's another entirely to question your own. That can't currently be explained by known, reputable science.
What's more, it's unlikely to be a "soul", because the computational parts of our brains are aware of its existence, they're integrated; yet there is in fact an observer present.