One can be an idealist without being a theist; Berkeley was a theistic idealist but there are other types, such as T. H. Green, F. H. Bradley and Bernard Bosanquet.
It's a lot more difficult than you realize to avoid intellectual conceit. The fact that you say you KNOW Christians (and atheists) can't make the kinds of claims they think they can make strikes me as if you are claiming to know something that everyone from Bertrand Russell to C. S. Lewis to Keith Parsons to Victor Reppert to Jason Pratt to Steven Carr has failed to figure out.
A lot of your reactions to issues strikes me as a re-action to "Christian fundamentalism." I spent a period of my life reacting to what I thought was "fundamentalism" myself. That is why, long ago, I went to a liberal seminary instead of a conservative one.
When you read something written engaged in apologetics and you say "That person is an apologist, so they must be saying this, even though they really didn't say it, because that is what apologist always do," I maintain that you are reacting like a fundamentalist.
You may not know what the truth is, but you are sure that evangelical-friendly people from McDowell to Lewis are full of baloney.
What I actually said, of course, was not that you were a fundamentalist, but that fundamentalists (or Christians who are caught up in the vice of fundamentalism), often talk about "vain philosophy."