If we're willing to call McVeigh and his ilk "not true Christians," then we must be willing to accept the notion that moderate Muslims might feel the same way about al-Qaeda.
I don't think there are as many moderate Muslims, but the ones there are have every right to disassociate themselves from the nutjobs. And we ought to be encouraging all Muslims to do so.
I think there are quite a few moderate Muslims, but they have been largely intimidated into silence, or just otherwise angry at the West for a variety of reason, so they don't speak out as often as they should (fear and a desire for petty revenge; a winning combination!). We should be encouraging Muslims to speak out. And I think more will as time goes on. I'm expecting a major Shiite Sunni split soon.
But when I say McVeigh is not a true Christian, I really mean it. They are neo-nazis who espouse a few choice tidbits of Christianity and leave the rest as BS.
Don't be a fool.