How can you be sure that God doesn't exist?

[Mister Agenda]

Atheism isn’t a claim that gods don’t exist.

It is a rejection of theistic claims, however.

I’m not going to claim that there are no gods, nor am I going to accept the claims of people who say there are and that these gods speak through them, or the claims of books that show that the authors had little or no understanding of how the world works.

If a deity wabts me to believe in it then it can come and introduce itself in person.

Notice I said “believe” and not “worship”

I hate the suffix of any "ism" being attached any position.

All an an "ism" means is that the person believing it claims it.

An 'ism' can also indicate a state of condition, like albinism or metabolism. The 'ism' of atheism and theism is like that: theism is the state of holding a belief that at least one god is real, atheism is not being in that state.

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As to the OP, I do not need to know with utter certainty that Bigfoots don't exist to not believe that Bigfoots exist. I AM utterly certain that the God of Abraham who flooded the world to the mountaintops, stopped the sun in the sky to facilitate a battle, and afflicted Egypt with plagues is not real because those things did not happen.