If you want to say that all the world's religions are divinely inspired, and that God is trying to communicate with human beings in many different ways, then that's an interesting view, but it's some kind of New-Age religion, and it has nothing to do with what I'm talking about here. The fundamentalist Christian philosophers I object to are all exclusivists. That is, they think that the Bible and the (official) Christian tradition alone are divinely inspired. It's this view that I consider to be irrational.
I'm not sure how you are defining irrational. In my opinion, belief in the Great Pumpkin is more rational that belief in the Invisible Pink Unicorn (bless Her holy hooves) who is simultaneously invisible and pink.
Belief that Christianity is the only divinely inspired religion is comparable with belief in the Great Pumpkin, but belief in the Trinity is comparable with belief in the Invisible Pink Unicorn. (I guess the Christian philosophers distinguish between person and substance, but I suspect the followers of the Invisible Pink Unicorn might have similar ways to finesse the irrationality.)
Almost every Christian denomination officially believes in the Trinity, so liberal Christians should not be spared. On the other hand, belief is not binary 0 or 1; it is a real number 0 through 1. The more irrational concepts might not be strong beliefs, so we shouldn't automatically declare the believer to be irrational for accepting them as part of a package deal.