I think people join religions as much as for social benefits as for religious belief. As a Muslim I have to admit to that. Because religions as actually far more like trade unions than tribes: the latter is based on ties of kinship whereas the former are based on common purpose.
Think about it. With their vying for members, each claiming to be the best at serving your needs/rights, each having different responsibilities to Co members, some having common liability all for one one for all, some not, some requiring obedience to powerful leadership figures, some not, some more accommodating to establishment than others, some less, eg BMA vs RMT, many doing outreach work outside of their trade union but largely their activities and advocacy is for the benefit of their own members. And likewise people fall in rank and file because the benefits of membership outweigh the negatives, eg going on strike when you would just like to go into work because you have to support the purpose of the union, paying in membership fees, etc. In exchange you get the help and advice of your trade union rep, sometimes they tell you you have a case other times they say buck up, you get membership benefits, burial fund, insurance, subsidised holiday homes.
And atheists are like people who don’t want to join a trade union for any or all of those reasons, like colleagues who don’t join a union because they want to go it alone, don’t want to be beholden to a group, don’t want to strike when they’ve don’t see the point of it, don’t want to pay in, etc. But they still largely benefit from the rights that trade unions fought for, (even though their power, like the church, is waning now). And people don’t see the point as much because we already have the employment and legal rights established in law no need for trade unions anymore. but in some parts of the world, trade union membership is still crucial to get on and not be abused in the workplace, just as religious membership is subscribed to for those same reasons also. For the protection of a group that is not related to you by blood but has a mutual stake with you.