Materialism-the doctrine that nothing exists except matter and its movements and modifications.
If spirits and God and heaven and hell all exist wouldn't they be "moving" anytime a spirit went to heaven or hell or anytime God or his angels or the devil intervened in the world?
And then wouldn't that make them a form of matter? If it can effect matter then it must itself be matter.
I'm not saying all of those things exist or that they don't. I'm just pointing out that while materialism is true it is tautologically true. The mere fact that something exists makes it material.