You find a book just sitting there. No title. No dewey decimal number to tell you what type of book it is. It could be a novel. It could be a phone book. It could be a blank journal waiting to be filled. How do you decide what kind of book it is?
Well, you open it up and read it, silly.
If the first line is, "Once upon a time...." then you know instantly what type of book you're reading.
You know that it's a fairy tale. You know that the author wants you to know that it's a fairy tale. The author has giving you all you need to know about what type of book it is, with those 4 words. You and the author have established a code language. It's not exactly a "secret code," because there's no "secret." It's a simple code, and one which you and the author both agree on, instantly. The author is not going to recite a historically accurate occurrence. If there are dragons and trolls in the book, you aren't going to say, "Wow, this author sure is stupid to think that dragons and trolls really exist." That would be as silly as it would be for you to believe that dragons and trolls really exist, just because they're written about in the book.
Genesis 1 begins, "Once upon a time."
The language of Genesis is that of myth. One doesn't even really need to understand ancient Hebrew to get it. I understood it decades before I learned Hebrew, though when I did learn Hebrew in my 20s it became that much more obvious, in the original language, that it was myth.
To say that the Bible is myth does not deny its divine authorship. On the contrary, symbolic speech is evidence that we are more likely looking at divine truth. After all, do we not believe that Jesus used parables, and that Jesus is God? Jesus used parables, BECAUSE Jesus is God. That's how his disciples knew he was God, and how WE know he's God.
To deny the mythic quality of Scripture is as heretical as to deny the truth of it. It's like complaining that Shakespeare's plays are historically inaccurate, or arguing with historians and believing that Shakespeare's plays are historically accurate. Both are silly, and both show that you're completely missing the point of Shakespeare's plays.
Both atheists, who deny Scripture, and certain Christians who insist on taking it literally, are equally missing the point of Scripture, and have abandoned the words the Author gave us. They're both equally wrong. And they don't even realize it.