There are definitely Christian materialists and there are definitely Futurama-style religions that believe in a real, physical creator God somewhere present in the world. In terms of strict materialism, we "know" stuff like elementary particles have no internal states because they must be simple according to all of our current understanding of physics and all observations we have made, this rules out any actual interaction from the soul-realm to material (it would have to be one way the other way if mind-body duality is right) and would also rule out any material "soul" (there's nothing to support a material soul, you're body isn't any lighter when you die for example so "souls" must be massless and so on). Maybe it 's all a one-way recording to the spirit-realm, who knows.

Descartes definitely fucked up by leaping from "something is thinking" to "I am thinking" and yes, it was basically assuming there is something called I. The most he could have come up with is that "thinking is occuring." From whence thinking came, we can't really say from a radical doubt perspective.