Why is it that Christianity is so hugely believed as the absolute truth? Many
There are a lot of questions here, but not many of them are philosophical in content. Nonetheless, I'll have a shot.
Plainly there are many people throughout the world who subscribe to faiths other than Christianity, including faiths, such as Judaism, that are older than it.
As to the question about the Bible, I'm not sure what "questionable origins" it has. Last I checked, a great deal was known about who wrote its various parts, when, where, and under what circumstances. The fact that Isaiah, for example, was written by three (maybe more) different people at different times makes it no less interesting to me. In fact, it helps me understand it better. Perhaps a certain sort of fundamentalist would reject this kind of historical research or find it threatening. But not all Christians (or Jews, or Muslims, or...) are fundamentalists. Indeed, in Christianity, anyway, fundamentalism is a recent invention, and fundamentalists a distinct minority, despite the success they've had convincing so many people otherwise.
I'd suggest you have a look at Peter Gomes's The Good Book, if you're actually interested to learn something about the Bible and what it actually means to thoughtful, non-fundamentalist Christians.