What is an Antitheist? 

The Oxford English Dictionary, second edition, defines antitheism as opposition to "belief in the existence of a God."

The Chambers Dictionary has three definitions for antitheism: "doctrine antagonistic to theism; 'denial' of the existence of a God; opposition to God."
As atheist means no more than not believing in god, it's important as antitheists  to set ourselves apart. 

 To elaborate, what a person believes isn't an issue. Who are we to say what a person chooses or chooses to not believe?  If we were to act like thought police, we would be behaving little different to the dictatorial theocracies we find so distasteful. What matters is what a person does with their belief. When religion encroaches upon those uninvited and harms those not choosing to accept ridiculous religion and its perverted morals, the faithful then by default make religion everybody's business. 

We're a very accepting group and we welcome all. Antitheists care enough about the harm caused by religion to want to see an end to it. This joint aims means, unlike so many other atheist groups, we can boast and enjoy a certain amount of cohesion. As humanist antitheists, and unlike religion, we  believe all people deserve equal opportunity and dignity without discrimination. 

Joined with us in opposing religious harm we have: pantheists, atheists, agnostics, Pastafarians, nihilists, deists, LaVeyan Satanists, Pagans and even theists who agree with our ethos and aims. 

Theists declaring they don't agree with the noxious aspects of religion, are worse than those less aware of how harmful religion is.  The theists not supporting the hateful aspects of religion, do at least have the grace to recognize it for the hate filled organization that it is, and their version is definitely an improvement, but it's no longer the original religion they signed up for. Besides, how opposed to religious harm are theists, despite what they say, when they stand with their heads counted as in support?

Where no harm exists, what argument can there be?

If it harms none, do as you will, beyond neither harming or depriving others. 

Thank you for your support.

Peace and much respect.