Certainty of non-existence of God is Ideology.
Maybe, but that's not what atheism means to most atheists.
Atheism is not believing in God(s). This is different from both saying "I don't know", like agnostics do and actively denying the possibility of existence in God.
Most of us, when we say "I don't believe in X" are not saying "I have absolute certainty that X does not exist". We're saying "I've not seen any reason to believe in X, so I don't". I'm not agnostic about the existence of Santa Claus - I don't believe in Santa Claus. But maybe there really is some guy living in some secret compound on the North Pole -- it is however so exceedingly unlikely that I'm not going to go around answering "I don't know" if someone ask.
Most atheists are "soft atheists" who fall in the category above, who if asked will say "God does not exist," but who if pushed will concede that just like I can't know for absolute certain that Russell's infamous teapot or Santa Claus doesn't exist, I can't absolutely rule out the existence of every conceivable definition of a God.
And this is not an ideology any more than the non-existence of Santa Claus is.
There are also "hard" atheists that insists that God 100% certainly does not, or can not, exist, and in those instances you might be right to call it an ideology. But they're a fringe - personally despite growing up among atheists I've never met one in person.
Some atheists do have ideologies that are related to their atheism, many others do not.