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

[BrianSoddingBoru4]

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.

Really?  You object to words like 'socialism', 'capitalism', 'schism', 'militarism' and so on?

Boru