I don't believe in God and even if there would be a God, us people would never be able to really grasp the idea of something so big and we definitely wouldn't have stories like the bible or the quran telling us God's word exactly lol, those are fairy tales written to control the masses and giving absolute powers to the church and religious people. Wake up people, you don't need some book to be a good person.
The point isn't to grasp the idea of God. The point is to believe that He exists, and to believe what He says.
You said, "... us people would never be able to really grasp the idea of something so big," so why would we have or not have stories? If God is so big, can't he cause stories to exist among us if He wants?
How do we know if a story is a "God story?" We know it if we compare the story to things that happen naturally, and see if the story is something that couldn't happen without God's intervention. But it takes a bit of study if you want to do it by scholarly application.
Something as important as is understanding the Word of God is important, isn't it?
But God aside, how can we truly be sure to understand any word? From a empirical perspective all that I do is recognize patterns and respond to them. I have no clue what anything that you or any other person truly means when they use language and can only assume that you responding to patterns similarly as I would would mean that you have a similar experience of the world. That assumption is powerful from a utility aspect, but there's ultimately no basis to it. So if I wanted to believe that we derive the same (or even just similar) meaning from the same words I could only go off of faith.