Ancient manuscripts are pretty important, without them the world before the fifteenth century would be practically as dark to us as the world of the bronze age is. It is never possible to know anything for sure from a single manuscript, but when there are multiple copies or, better yet, different works that corroborate each other it's possible to have a clear image of the subject matter. That's how we know that the tradition of monotheistic worship stretches back millennia, and for Jews most of the basic practices connected with it are the same today.
Of course everything that survives is written by man, but it's all been copied from earlier sources.
How are you so sure? It's obvious that a lot's been corrupted over the millennia, critical biblical scholars are even able to identify certain portions with different writing styles and presumed author, but supposedly it all comes from the word of God. There are some factions that even think Moses received a scroll from God on Mount Sinai, along with the Ten Commandments and a significant body of oral law.
Any complete religious doctrine is built upon both faith and reason.
The problem with the Bible is that many things it says are demonstrably false in so many hilarious ways, which tends to destroy its credibility. Therefore, anything said in the Bible must be taken with a handful of salt.
As for the end, religions are basically a corruption of reason. They make a leap of faith, essentially, they require a belief in something without reason, and then try to build constructs of reason on top of that. The problem is, that a belief based on faith, no matter how much you use reason to try to back it up, is still not particularly reasonable or logical, since its basis is in faith.