How are Unbelievers, specifically Atheists and Agnostics, treated under an Islamic State?

I have always known that Ahl-Al Kittab (people of the book) are free to follow their religion according to their laws within the Islamic State respectfully while paying the Jizya. But what about Atheists and Agnostics and the like under an Islamic State? What laws do they respectfully have while they don't necessarily "believe" in such Divine laws as other religions. Do they have their own man-made secular laws, or automatically under Sharia Law (Which is impossible since They are kaffirs and Sharia is in direct conflict with them)? This goes to all beliefs out there (Communist, Pagan, Polytheist etc.)