Are you 100% sure God doesnt exist?
No but only in the same way I'm not 100% sure leprechauns don't exist. Atheism doesn't involve certainty. You were born an atheist until your parents no doubt lovingly forced a supernatural view on you.
If someone on the street walked up to you and said they were God, your default position is you don't believe them because there's no evidence. That is an atheistic response even though you can't be completely certain that person isn't actually the second coming of Jesus.