Religions are moral exoskeletons. If you live in a religious community, you are enmeshed in a set of norms, relationships, and institutions that work primarily on the elephant to influence your behavior. But if you are an atheist living in a looser community with a less binding moral matrix, you might have to rely somewhat more on an internal moral compass, read by the rider. That might sound appealing to rationalists, but it is also a recipe for anomie—Durkheim's word for what happens to a society that no longer has a shared moral order. (It means, literally, “normlessness.”) We evolved to live, trade, and trust within shared moral matrices. When societies lose their grip on individuals, allowing all to do as they please, the result is often a decrease in happiness and an increase in suicide, as Durkheim showed more than a hundred years ago.— Jonathan Haidt, The Righteous Mind: Why good people are divided by politics and religion, "Religion is a Team Sport."
Societies that forgo the exoskeleton of religion should reflect carefully on what will happen to them over several generations. We don’t really know, because the first atheistic socieites have only emerged in Europe in the last few decades. They are the least efficient societies every known at turning resources (of which they have a lot) into offspring (of which they have few).
While many European countries have low native birthrates, the successful ones have high immigration rates. I see this as a transitional phase in group selection. Haidt argues that religious communities and practices are a group adaptation: groups of humans with a strong religious bond are able to overcome free rider problems and outcompete—as a group—groups which are less cohesive or whose cultural practices are less effective at bringing collaboration to fruition. For most of human history, one's membership in a religious group was generally from life through death: leaving a religious group meant leaving a tribe, or having a conquering tribe's religious system forcibly replace the conquered tribes.
But now large group "superorganisms" (including religions, nations, governments, and companies) don't have to be tied to a human lifecycle. In the 21st Century, humans have considerable ability to move between groups. Much as an animal organism doesn't die as its cells come and go at a steady pace, a paper entity can grow and thrive so long as it can get a continual influx of new resources, even if those resources shift focus to providing outcomes beneficial to the group rather than reproducing on their own. This is particularly true for companies: two parents often work for different companies; a baby born to the couple is not generally part of either company's culture; and there's no assumption that the child will grow up to be part of the company as an adult. Workers might be part of a company for a few months or a few years (and rarely more than half their lives), yet companies like IBM and UBS are older than roughly half of the countries in the UN.