How do Christians know that the Bible is factual and not just a fictional piece of writing?

Face it, people were bored back then. Little entertainment and years of repetitive lifestyle prompted the motivation for creative writing, and some people could have written stories for entertainment. This doesn't primarily have to be for Christians and the bible, but also for other religions and holy books such as Islam and the Qur'an, Judaism and the Torah, etc. So is there a reason that people devoted their whole religion and lifestyle to one book, that has no proof of its events even being true?