If I'm right, then liberal, individualistic atheism is impossible as an organising principle of society because any doctrine that actually works to hold society together is indistinguishable from a religion. It needs its rituals and it needs its myths. A philosophy will grow around it in due course. Now perhaps you can have, at least on a small scale, a society committed to the principles of rational and tolerant disagreement and the sovereignty of reason. But what you end up with then isn't some rational Athens of the mind. It's Glastonbury.
As someone else recently noted—it slips my mind where, exactly—for people like the Atheism+ crowd, atheism naturally and logically entails progressivism. But most progressives do not see progressivism as naturally and logically entailing atheism, and any attempts to steer progressivism in that direction would likely be disruptive to the goals both groups hold in common. The argument that progressive actions are still dangerously contaminated by even the most benign presence of religious sentiment will seem like dogmatic purism to people who care more about practical results, which is why it has been suggested that you leave it out of your social justice organizing. One national rally and some encouraging poll numbers does not mean you're ready to remake the world in your image. Many progressives, if not explicitly Christian to begin with, are at least happy to portray Jesus as the original hippie and pay lip service to the importance of emulating him.
I agree that this is tendentious history, to say the least. But I can say that because I'm apolitical and somewhat misanthropic, and I can afford to be a stickler for accuracy over utility. If I were possessed of a burning desire to right some societal wrongs and raise up the downtrodden, I would have to admit that Hippie Jesus might make for a useful figurehead. A thoroughly secularized religion of the COEXIST variety may in fact be the best way of binding a society together in a useful way while leaving enough freedom for purely godless individuals to hold themselves apart philosophically and pride themselves on it.