It's so obvious, or is it dogmatism? The Presumption of Amatterism

One of the differences you will notice in reading the writings of C. S. Lewis, if you are someone familiar with present-day philosophy, is the extent to which, in his time, Idealism simply had to be taken seriously. When Lewis accepted the Argument from Reason, he went, not to traditional theism, but to Absolute Idealism. 

At one point, philosophers not only were idealists, but thought it was obviously true, as Ed Feser points out in an entry from 2009. 

But has idealism been disproved? Consider this comment from a commentator at Debunking Christianity: 

I've been hanging around Mr. Loftus' Debunking Christianity for many years, and now my default position is that Christianity is thoroughly debunked since no Christian can show any Christianity-specific claim to be true. 

Hmmm. A position is fully and completely debunked if no one can demonstrate that any of its basic posits are true. 

Well, how about the existence of matter, the central posit of philosophical materialism. Has anyone refuted idealism, which denies that matter exists? So, by this reasoning, materialism is debunked, since no one can prove that matter is real. If we ought to accept the presumption of atheism, the view that we should be an atheist unless we can prove that God exists, shouldn't we equally accept the presumption of amatterism.