I know this thread is from a couple of years ago, but I found myself reading through the whole thing, and it has moved me to post a couple of thoughts.
Firstly, in an odd bit of synchronicity, I'm currently reading Gibbon's Decline and Fall of the Roman Empire, and at just the time that I was reading this thread I had just read the section on schisms in the early church, in which he discusses how the Donatist heretics "asserted with confidence, and almost with exultation, that the Apostolical succession was interrupted; that all the bishops of Europe and Asia were infected by the contagion of guilt and schism; and that the prerogatives of the catholic church were confined to the chosen portion of the African believers, who alone had preserved inviolate the integrity of their faith and discipline", going on to talk about how tiny sects that schismed from the schismatics "could affirm, without a blush, that when Christ should descend to judge the earth, he would find his true religion preserved only in a few nameless villages of the Caesarean Mauritania".It's hard to avoid the comparison with tiny churches in the US or elsewhere today who believe they're the only ones going to heaven come the end times. Some things don't change.
However, there's a view that seems to be repeatedly expressed in this thread, without any real challenge, that this is the general belief of the world's many churches - that only they are the saved. This is simply untrue. I've no idea if it's true of a majorty of denominations, since I'm not familiar with the doctrines of thousands of different sects, but it's certainly not true of the majority of believers. And I don't mean this in the sense of individual worshippers contradicting the official policy of their church - most major denominations do not teach that only their faithful are going to heaven. Even the Catholic Church has now accepted that there are other ways to acheive salvation, a view they share with the majority of Anglican, Methodist, Reformed and Presbyterian churches.
People like to worship God in a way they feel comfortable with, which fits in with their culture and inclinations. This doesn't mean they believe this is the only acceptable way.