The Pew organization, which certainly has no bias that I can detect against religion, had reanalyzed some data from its 2014 U.S. “Religious landscape study,” asking people who said they were both “nones” (those not affiliated with a church) and also had formerly been raised as church members but later abandoned that membership. The results are described here, and the methodology (apparently a phone survey of 5,000 people) here.
What they did, as you can see in the chart below, is divide those who abandoned their childhood faith into five groups based on the reasons for their apostasy. To wit: don’t believe in religious claims, dislike organized religion in general, those who are “spiritual” or “seekers” and are classified as “religiously unsure/undecided”, and those who still believe but are too busy to do the church thing. Each of these five, given as a percentage of the total, is in bold in the first column below, and then within each group the reasons are further subdivided (still first column):

Right off the bat you can see a problem: these reasons are overlapping, so how did they group people into categories? Further, the numbers in bold in the first column don’t add up to 100%, as they should (they add up to a bit more than 103%).
Well, okay, there are some problems, and there are also problems of self-report. That said, we can still get something out of the data above. The main lesson, which probably isn’t an artifact of self-report, is that 49% of people say they left their childhood faith simply because they no longer believed in the claims of that faith.
Pew also gives a table of quoted reasons for people falling into each of the five categories (below), and add this in the report:
About half of current religious “nones” who were raised in a religion (49%) indicate that a lack of belief led them to move away from religion. This includes many respondents who mention “science” as the reason they do not believe in religious teachings, including one who said “I’m a scientist now, and I don’t believe in miracles.” Others reference “common sense,” “logic” or a “lack of evidence” – or simply say they do not believe in God.
Those who claim there’s no conflict between religion and science now must tell us why learning science drives people away from religion, and I don’t see how they can do it except by accepting my thesis in Faith Versus Fact: science and religion both make statements about how the cosmos is, but only science has a way to test those claims. And by instilling the habit of doubt as part of its toolkit of understanding the Universe, science automatically leads to weakening religious belief, which, after all, rests on no evidence at all but is simply fabricated wish-fulfillment and a means of social control.
Here are some representative quotes. At the FFRF meetings in Pittsburgh I’ll talk about why evolution in particular tends to turn people into nonbelievers.

Finally, they divided members of each of the five classes as to whether they considered themselves atheists, agnostics, or “nothing in particular”. Again this is a problematic classification if based on self-identification, but does show strong associations with reasons they left the church.
As Pew says:
Religious “nones” are by no means monolithic. They can be broken down into three broad subgroups: self-identified atheists, those who call themselves agnostic and people who describe their religion as “nothing in particular.” Given these different outlooks, it is not surprising that there are major gaps among these three groups when it comes to why they left their childhood religion behind. An overwhelming majority of atheists who were raised in a religion (82%) say they simply do not believe, but this is true of a smaller share of agnostics (63%) and only 37% of those in the “nothing in particular” category.
In fact, while this latter group certainly includes many nonbelievers, it also has substantial shares of people who, alternatively, are opposed to organized religion (22%) or who could be described as religiously unsure or undecided (22%). And more than one-in-ten people with the “nothing in particular” label (14%) say they are either non-practicing or too busy to engage in religious practices, compared with zero atheists in the survey and only 3% of agnostics.
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More heartening news, this time from Britain. In an article in the “This sceptic isle” section of the Economist, the always anonymous writer argues that “Britain is unusually irreligious, and becoming more so. That calls for a national debate.” First, the heartening facts—to nonbelievers, that is:
Last year the church reported a “sharp upturn” in such disposals [churches getting sold off because there aren’t enough parishioners to support them]. That hints at a milestone that Britain reached in January, when figures for weekly church attendance fell below 1m for the first time, as well as one passed in 2009, when the proportion of Britons saying they had no religion (49% in the latest data, for 2015) overtook that saying they were Christian (43% in 2015) in NatCen’s British Social Attitudes survey. Other figures also point to this spiritual sorpasso: since 2004 church baptisms are down by 12%, church marriages are down by 19% and church funerals by 29%. A 65-country study by WIN/Gallup last year found a lower proportion of people are religious in Britain than in all but six other countries.
The country is littered with evidence of the change. Everywhere deconsecrated churches are reopening as bars and restaurants. Five hundred churches were turned into luxury homes over five years in London alone. Shrinking congregations and growing repair bills are typically the fatal combination: about a quarter of Sunday services are attended by fewer than 16 parishioners. The Church of England is doing its best to manage this trend. Christmas-only parishes, catering to the once-a-year crowd, are one avenue. A new app enables cashless millennials to chip in to a virtual collection plate.
All this despite the fact that the percentage of Brits who describe themselves as “religious” remains pretty constant: about 80%. But it’s clear that they’re religious in a different way—a way verging on nonbelief. Britain is in fact is becoming very secular very fast, and faster than the U.S.
Sadly, the article then devolves into a soul-searching discussion of “how can we possibly replace religion?” The author tortures herself with thoughts like “What will we do with the Bishops in the House of Lords?”; “Who will give us a place for moral guidance and communion?”
The Economist fails to consider that we don’t really have to worry about these matters. The lesson of other secularized societies, including France, Sweden, and Denmark, is that religion gets replaced by a natural social evolution that somehow meets the needs of former believers. In fact, as society improves and becomes more empathic towards its most deprived and despised, the need for religion largely vanishes. All the Economist‘s soul-searching, and its claims that Britain must “lead the way” in helping the world secularize, is just so much hot air.