Excellent thread by the way!
But what is the evidence for materialism? Presumably, that scientific investigation reveals the existence of nothing except material things. But religious believers will plausibly reply that science is suited to discover only what is material (indeed, the best definition of “material” may be just “the sort of thing that science can discover”). They will also cite our experiences of our own conscious life (thoughts, feelings, desires, etc.) as excellent evidence for the existence of immaterial realities that cannot be fully understood by science.
At this point, the dispute between theists and atheists morphs into one of the most lively (and difficult) of current philosophical debates—that between those who think consciousness is somehow reducible to material brain-states and those who think it is not. This debate is far from settled and at least shows that materialism is not something atheists can simply assert as an established fact. It follows that they have no good basis for treating the existence of God as so improbable that it should be denied unless there is decisive proof for it. This in turn shows that atheists are at best entitled to be agnostics, seriously doubting but not denying the existence of God.
And as a starter for that subject, if anyone is interested in learning more, I highly recommend Victor Reppert's book, "C.S. Lewis's Dangerous Idea." A real thought-provoker of a book!