Because the religion that I was taught from the age of 3 or 4 told me so - for about 45 years they told me so.
Because if someone is going to base one's entire god-concept on the only book that explains him, and is claimed to be 'his book', and claims to show us how to worship that god-concept - it seemed entirely consistent to believe it was not just a bunch of myths and legends [translated bullshit to those with a meter].
But, be assured I have gotten over both reasons to take either the god-concept or the book that supposedly explains him with any seriousness.
Thanks Jeff, I appreciate your reply.