It seems reasonable to regard religion as the proximate method of group selection in humans - in other words, when humans groups compete and evolve group adaptations, these are instantiated by means of religions. Individual humans have adapted to live in a context of religion; and when religion is absent, human behaviour becomes maladaptive - because human instincts are 'designed' to function in a religious environment.
It has long been a consensus among (secular) social theorists that the main (secular) function of religion is social cohesion - that is, religion can enable larger and more complex forms of social functioning; including the stimulation and enforcement of motivation, altruism, long-termism.
Since all humans - until recently in The West - have evolved in the context of religion; therefore religion must, over multiple generations, have had gene-selective consequences that shaped individual instincts and behaviours.
Group selection entails that group behaviour be referenced to something outside the group. This is what groups cohere-around, organize around, cooperate to promote. Throughout history this 'something' has been religion - variously the spirits, the gods, or One God.
Since this has apparently been the case throughout all known history and stretching back into pre-history, individual humans have evolved to be coordinated by religion: religion is a built-in, innate expectation for each individual human; and if religion is absent, then individual behaviour lacks a necessary adaptive context.
Having been under group selection for so long, where each individual functioned as a 'component' part of a religious society; then individual level instincts will relatively have atrophied. So, remove the religion from an individual human being, and the behavioural rules and patterns lack context, and are maladaptive.
Individual humans absent religion are un-equipped to pursue their own reproductive success.
Religion is therefore the medium for, and regulation of, altruism - which is the propensity of individuals to sacrifice their own short-term comfort and pleasure, health, survival, and ultimately their reproductive success, to that of the group.
In sum: Humans just are adapted to serve the group via religious structures which reference individual behaviour to some-thing outside the group.
Clearly, 'the group' in group selection will be bounded - and cannot be scaled up or down, made larger or smaller, indefinitely; since there must be mechanisms for rewarding group-helpful, and suppressing group-harmful, behaviours - and such mechanisms (like status or material rewards, shaming or physically-coercive sanctions) differ between religions, and these do not scale indefinitely in either direction.
Group selection is strong: it must be stronger, in significant respects, than individual selection: group selection must be strong enough to overcome individual preferences.
This means that group selection operates to affect the nature and strength of individual preferences - individual preferences have until recently always operates in the context of religiously-mediated group imperatives; because, over many generations, selection will (overall) tend-to mould individual preferences significantly to serve the needs of the group.
While group selection has been significant on all humans everywhere and at all times; European populations (also probably East Asian populations, perhaps to an even greater extent?) have been strongly group selected for large-scale cohesion over many dozens of generations; so that the effects of group selection are more significant in those of European descent than in most other populations.
This implies that the selection effects of religion on individual behaviour has been more significant in those of European descent than in most other populations.
European populations had Christianity as the proximate mechanism of group selection for hundreds of years; shaping the instinctual basis of the individuals. And Christianity must have been an extremely powerful mechanism of group selection, because it enabled what are, by world historical standards, very large cooperating groups persisting over multiple generations.
(By contrast, simple animistic religions are able to enforce cooperation of some scores of people; more complex totemistic religions can enable the cohesion of thousands; and the complex and literate Temple religion of Ancient Egypt enabled some millions of people to cohere for three thousand years! Christianity seems to have been similarly powerful to the ancient Egyptian religion, sustaining complex cooperation among millions of people.)
It may therefore be assumed that the people of Western nations inhabited by those of European descent evolved to become extremely dependent on Christianity in order to be adaptive.
These same people, with the same instincts that operated within a strongly Christian context for so many generations, now find themselves in a society from which Christianity has been (all but) deleted.
Individual behaviours now have a very different environment in which to operation - indeed, the environment is in multiple respects and increasingly anti-Christian. Little wonder that grossly maladaptive behaviour is currently rife - indeed mainstream.
Sans religion, Western populations lack the instinctual basis for individual level survival and reproduction, since these instincts long since atrophied - and atrophied to a more extreme degree than in most populations.
Now basic instincts such as reproduction, group survival, defence, long-termism for group goals, self-sacrifice for the group are defective or absent. The interaction between individual instincts and the non-religious environment is producing multiple, population-lethal pathologies. Voluntary subfertility is nearly universal; native population-replacement is advocated and celebrated; maladaptive forms of sexuality are common and actively-promoted; self-mutilation is escalating, normal and admired; the clamour for on-demand, comfortable, 'assisted' suicide grows greater with each year...
Living without religion, but with a genetic makeup that had evolved to assume religion, to expect religion, and to live-within religion; the instinctual basis of Europeans, including their powers of evaluation and judgement, are revealed as both ineffective and inappropriate.
Of course, under current conditions and continuing present trends; eventually and after many generations individual level selection may lead to the evolution of new effective and appropriate instincts that aimed-at individual survival and reproduction - then, presumably, large scale societies would break down into much smaller competing units, since such individuals would have evolved to be resistant to the social cohesion mechanism of religion.
Alternatively, the adaptiveness of European populations may be strengthened by the restoration of Christianity.