Don't just rely on wikipedia for your definitions
A delusion is also a false
opinion.
Atheism is the
opinion that there is no theistic god.
Atheism is irrational, based on nothing other than opinion
Prove to me otherwise. Give me a factual, evidence based reason for atheism?
Your premise is flawed: atheism is the stance whereby one is not a theist. A theist is one who affirms the existence of deities, and, thus, an atheist is one who
doesn't affirm the existence of deities. The atheist can be someone who actively affirms that deities don't exist,
or someone who affirms neither the existence nor the non-existence of deities - in both cases, the person is not a theist, and therefore qualifiers for the moniker of 'atheist'.
What you are arguing against is what's called
Strong Atheism - the position that deities do not, in fact, exist. You can accuse them of having faith and of having no evidence for their claims (the irony of such an accusation notwithstanding), but don't fall into the equivocation trap: not everyone who's an atheist affirms that deities don't exist.
And even if you hold some minority definition of atheism, it still remains that the vast majority of those who
call themselves atheists use the above vocabulary, and moreover would specify themselves as
Weak Atheists (that is, those who neither assert God exists, nor that he doesn't) - therefore, demanding proof for the non-existence of God from a self-professed atheist is only going to bemuse the atheist: odds are, they don't, in fact, assert God doesn't exist.