Interesting point. I was brought up as an atheist, so I had no reason to believe in any God. Nonetheless, I thought about it later.
This was my main reason when I was a kid:
1) Creation goes against evolution, therefore Christianity and the concept of a God must be false.
I also observed:
2) Lack of interference. You don't see God. You don't see God interfering in the world. Is this a God that doesn't care? Are we suffering for sin? Is it my problem what my ancestors did then?
Later came:
3) Lack of uniformity. If there is a one true God it's only reasonable everybody believes in the same God. It rains devine scriptures often and everybody can read. No one has eye problems or a mental disability. God is accessible and we can all know God.
4) Historical discrepancy.
My ancestors were pagans or believed in the Germanic Gods. Since this pre dates Christianity it's only reasonable the Germanic gods were the true Gods. But then, they did nothing against the Christian invasion. That does not speak for them. But then, if Christianity is real, why was Europe not given access to this right away? This Christian invasion looks human, not divine. The one true God should have been right by our side from the beginning.
5) Nothing would indicate the humans are the chosen ones. Humanity is 3 to 8 million years old. Some sharks and crocs are a few hundred million years old! If an animal has the chance, you will get eaten. Also what is that vast space called the universe doing out there? Religion makes it sound like humans and earth are the core of it all.
I can go on, but then I already have 5.
