In the passionate world of American atheism, the venom usually directed at believers has now been turned against the wrong kind of atheists.

The cause of this freethinking furore? A new movement called Atheism+. According to its website, "Atheism+ is a safe space for people to discuss how religion affects everyone and to apply skepticism and critical thinking to everything, including social issues like sexism, racism, GLBT issues, politics, poverty, and crime."

A+ was born when Freethought blogger Jen McCreight (the mind behind Boobquake) made a passionate call for a "third wave" of atheism, one that extends atheist activism into progressive politics and calls for a part of the movement to be one where women can exist free from the harassment that has plagued women publicly involved in the atheist movement.

The founders of Atheism+ say clearly that "divisiveness" is not their aim, but looking through the blogs and voluminous comments in the two weeks since A+ was mooted, trenches have been dug, beliefs stated, positions staked out and abuse thrown. A dissenting tweeter is "full of shit", while, according to one supporter, daring to disagree with Atheism+'s definition of progressive issues and not picking their side makes you an "asshole and a douchebag".

It took 700 years from Constantine renaming Byzantium in his own honour to papal legates circulating letters of anathema that split the Roman and Orthodox churches. Atheism, in its public, online life, has started exchanging internet anathemas – perhaps we should call them inathemas – in little more than a decade.

People are being told to wipe the spittle off their chins, take their heads out of their asses. The Life of Brian's lines about the various fronts for the liberation of Judea are being oft-recycled. 140 character brickbats are being thrown on Twitter under #atheismplus.

PZ Myers, soft-spoken in person but trenchant in print, said of A+ critics:

"It really isn't a movement about exclusion, but about recognising the impact of the real nature of the universe on human affairs. And if you don't agree with any of that – and this is the only 'divisive' part – then you're an asshole. I suggest you form your own label, 'Asshole Atheists", and own it, proudly. I promise not to resent it or cry about joining it. I just had a thought: maybe the anti-Atheist+ people are sad because they don't have a cool logo. So I made one for the Asshole Atheists:

A*

"

Fellow Freethought blogger Richard Carrier goes further. When one commentator suggests "atheism does not have the luxury of kicking people out of its movement", Carrier gives him a rare old quilting in most splendid prose:

"Yes, it does. Atheism+ is our movement. We will not consider you a part of it, we will not work with you, we will not befriend you. We will heretofore denounce you as the irrational or immoral scum you are (if such you are). If you reject these values, then you are no longer one of us. And we will now say so, publicly and repeatedly. You are hereby disowned."

How like Pope Leo's letter to the patriarch of Constaninople in 1053 accusing him of "many and intolerable presumptions, in which if – as heaven forbid – he persist, he will in no way retain our peaceful regard". Even at this most serious moment for the future of Christianity, the pope managed to resist the urge to call the patriarch immoral scum, an asshole and a douchebag.

One of the joys of atheism's outlets on the internet was that they were clever, deft, funny, tolerant and irreverent. It was certainly robust and not for the faint-hearted.

Those of us who do not wish to extend our atheism into someone else's definition of progressive politics may take rather unkindly to being described as immoral scum, useful but unsavoury body parts, and outdated contraceptive devices. In the week when American atheism made its appearance in the Economist's editorial pages, it seems to have been sowing the seeds of that most religious of events – a schism.

St Paul would be laughing his head off, had a Roman soldier not already deprived him of it. "See," he might now write after reading those modern epistles, the blogs, comments and tweets around the birth of Atheism+, "how these atheists love one another."