So let me get this straight... this is how we know for sure that the Quran is from a god:
1. The Quran tells stories about figures and characters from the old testament, combined with the fact that Arabs never knew how to write, therefore the Quran is from a god.
Huh, that's interesting. Not only because Arabs actually did have a writing system since at least the 4th century (It was Muhammad himself who was illiterate not Arabs as a whole), but because a society can still learn about the stories of the Jews and Christians from purely an oral tradition.
You know how this argument could be used in favour of the Quran being from a god? If the Quran was revealed to a group of people who had no means of knowing the Jewish and Christian stories like if the Inca in South America came up with Islam as opposed to Arabs.
2. The Quran says the Earth is round rather than flat, therefore the Quran is from a god.
The idea of the world being round goes as far back as 3rd century BC from a man living in Alexandria named Eratosthenes. Also, the Quran doesn't say the earth is round or even shaped like an ostrich egg (that is a modern interpretation with no tafsirs to support it). The Quran talks about the earth being made like a spread out carpet. Even prayer wouldn't make sense on a sphere-shaped planet.
A qiblah can only exist if the planet is flat, otherwise you aren't actually facing Mecca, but rather towards outer space.
3. The Quran civilized the Arabs, therefore it's from a god.
You could argue that Islam (Muhammad's wars) unified the Arabs, yes. The Arabs became civilized after conquering other already civilized peoples like the Persians and Romans and adopting their civilized lifestyles. Even still are you going to argue that because something led to civilization it's from a god? How many civilizations came to be that were polytheistic? Surely you don't also believe in their religions too.
Kafir
At the end of the day, We make choices. I make the choice of believing in God, the creator of our universe. You choose not to believe. To each his own.