… all things are composed of material and all phenomena (including consciousness) are the result of material interactions; therefore, matter is the only substance (from Wikipedia).
This belief is obviously anathema to theists, because it defines out of existence any notion of a supernatural God.
It also has its share of philosophical problems. It’s an easy target for philosophical deconstruction and reasonably easy to paint as irrational.
For this reason theological arguments often conflate materialism with atheism, because the philosophical issues with materialism can be used to conclude that atheism is also irrational.
In fact, most arguments of the atheism-is-irrational variety boil down to this particular point.
But the argument fails right out of the gate. While all Philosophical Materialists are atheists it obviously doesn’t follow that all atheists are Philosophical Materialists. That’s a simple rookie error.
Like most atheists, my position is simply that theism has roundly failed to make its case.
I don’t reject religion because I think the material realm is all there is. I reject religion because once you scratch the surface of any religion it becomes clear it’s been created by humans.
Of course I can accept the possibility that there is something beyond this physical realm. I just don’t claim to know anything about it.
And here’s the thing: unless you can present some pretty extraordinary evidence to the contrary, you shouldn’t claim to know anything about it either.