First, an atheist:
Religion is not the only force for group-cohesion, even if it has the advantage of having sacred spaces, authority, and thus loyalty (what Haidt identifies as primarily conservative values). I believe that care, a concern for fairness/ justice, and a sense of liberty (what Haidt identifies as what liberals tend to prioritize) are means to creating community as well. We do not need to give up a concern for what is true (a value Haidt does not list, interestingly, especially because it is a high value for many new atheists, including myself) in order to create shared group identities.
Contra Marx, he seems to see religion as a healthy prescription for all human culture. Flattering as this may seem at first, religious individuals will do well to note—and emphasize—that it is the truth of religious claims, not their social utility, that makes religion valuable. Religion starts with a divine being’s claim about himself; humans then relate to that divine being, to each other, and to nature based on the nature of this claim. Humans do not relate to each other in certain ways, and then make up a religion to explain their behaviour.