Well, if it's not literally true, and most of it seems to be mythological in origin, what's the point in trusting any of it at all? Yeah, I realize there are some good bits of general moral advice in there, but there's a lot of morally disgusting crap too, which God should have edited out of his "holy" book. The Bible promotes slavery, sexism, homophobia and animal sacrifice! And that's just in the first couple books... As far as I'm concerned, any book that promotes those things is not a reliable guide for morality.

If I'm capable of going through the Bible and picking out the grossly immoral bits myself, what's the point of using the book in the first place? That means I already have a functioning morality, which I am using to judge the passages myself, and which seems to be vastly superior to the type of morality that the writers of the Bible had several thousands of years ago. It's an interesting historical document, sure, but there's little evidence that any of it is actually true and even less evidence that it is a reliable guide to morality.

If you're going to go through and cherry-pick the good bits to follow while ignoring the bad ones, you might as well use Mein Kampf as your guide to morality. You'll be throwing most of that book out the window too.