'I'm 100% sure God doesn't exist "
This is completely fine to say. It's just as acceptable to say that as saying "I'm 100% sure god exists."
'everyone that believes in God is a delusional idiot'
Only people that can't accept that there are other cultures and beliefs than their own say this.
You are right, I didn't express myself the right way.
You can believe that something is true but that doesn't make it true.
You can say that you are certain without being certain.
People said 'I'm 100% sure that the earth is flat', then a genius came and told em all to fuck off---> their certanity was proven wrong.
This can happen with the question whether God exists or not.
About the 2nd part of your reply.
A lot of atheists are like that, and you can see it in this topic.
They are so sure about their own genius brain that they tell everyone else to fuck off.
most of the atheists I know are ignorant and mean as fuck.