The quadrupling of the market share of adults who are non-religious from 5% in 1972 to 20% in 2012 accompanied by a 50.2% increase in population over that time period, means that the number of people who identify as non-religious in the United States has increased sixfold in absolute numbers over 40 years.
This is an annualized growth rate in the number of non-religious adults in America of 4.6% per year over 40 years, and it has been fairly steady. The change in the market share of non-religious people doubled in both the first twenty years and in the second twenty years, so the growth rates for both half samples are almost identical.
About 60% of non-religious people in America today were raised in religious households and personally decided to set religion aside.
Most religious denominations have substantial corps of full time professional missionaries and clergy, in addition to well organized volunteer organizations involving a substantial share of their active membership, substantial institutional resources of money and property, well honed sales pitches that have been decades or centuries in the making, and the advantage of starting out with a favorable status quo.
There is no corps of professional missionaries out there actively trying to convert people to be non-religious. There are very few institutional resources or organized groups of volunteers devoted to promoting or maintaining a specifically non-religious lifestyle. There are a few small secular community organizations, but almost none have more than 100,000 members within demographic that includes household with about 63 million Americans. There are no well established scripts for the narrow task of encouraging people to become non-religious, or the broader task of living a non-religious life.
Stigmas are waning but remain real. Legal protections of the rights of the non-religious have been on the books since the late 1700s, but have had practical effectiveness until the Warren Court of the 1970s.
Instead, this is a trend driven almost entirely by the force of pervasive ideas that suffuse our culture.
Mostly, secularism in the United States has been a grassroots movement of atomized individuals making personal choices that make sense driven by the force of ideas more than anything else in an increasingly scientific, global, tolerant, feminist, inclusive society in which the message of religious organizations are no longer credible and now seems more immoral than moral on key matters like the role of women and gay rights. It also is taking place in the context of a society in which all institutions of civil society that require personal involvement have been eroding. A fair amount of the shift is also a reaction and counterrevolution of ideas in response to increased political activism by conservative Christians.
The percentage of Americans who are non-religious will almost surely grow as young adults aged eighteen to twenty-four, a third of whom are currently non-religious, grow older and as the trend reaches a tipping point. Another doubling in market share from 20% to 40% in the twenty years between 2012 and 2032 would not be out of the realm of possibility, although that may be a bit high. Any model of growth in market share of anything needs to assume a logistic curve rather than exponential growth, but S shaped logistic curves tend to be steepest in the mid-range of percentages, so there is no good reason to think that the next twenty years will show substantial slowing of this trend.
About four times as many kids will grow up in non-religious household as did among people who are currently adults, while all of the factors that led 13%-14% of Americans who were raised in religious households in the last four decades to become non-religious will only grow more powerful. People making that transition now face less of a social stigma, have more role models for living their lives, have more fellow travellers to provide peer support, and live in a world in which there is greater scientific consensus, there is a more global outlook, there is profoundly greater tolerance of gay rights, and gender equality is better established, than the world in which today's adults came of age. It wouldn't be surprising to see as much as an 8%-10% shift in market share attributable to these factors over the next twenty years in addition to baseline established by today's young adults.
My children see this in the schools within the Denver Public School's district that they've attended in which most parents are non-immigrant, white liberals, who Generation X or younger, a population in which more than 40% of the population national is non-religious. They have many peers who are either non-religious or who have a religious affiliation but aren't very actively involved in attending church or other religious activities. Certainly, they know people who attend some kind of Christian church every week and are active in church youth groups and all of their relatives in my parent's generation are actively practicing Christians. But, they know almost as many people who are actively practicing Jews as they do who are actively practicing Christians and they know or at least know of a few classmates who are actively practicing Muslims or actively practice Eastern religions.
This growth in the non-religious population is coming and will continue to come mostly at the expense of infrequently church attending Christians, particularly non-immigrant Catholics and mainline Christian churches. The residual Christian population will include a larger share of immigrant populations, more socially conservative, more theologically evangelical and more doctrinally orthodox Christians. The emerging red state, blue state regional and urban-rural divides in religiosity will grow greater. Forty percent of liberals are non-religious, while only nine percent of conservatives are non-religious. Liberals in the next generation will be far more likely to have grown up in non-religious households and will be far more likely to be receptive to the ideas that have driven so many liberals relative to conservatives to deconvert in the first place.
Twenty years from now, a majority (perhaps even a substantial majority) of white and Asian Democrats will probably be non-religious, while only a distinct although growing minority of Republicans will be non-religious.
Establishment Christianity as a baseline default assumption involving affiliation with formerly established national churches is disappearing. The Christians who remain, in a context more similar to the early Christian church than last thousand years (ironically, with the exception of the pervasively evangelical Christian South where local denominations mostly strive to emulate the early Christian church). They are once again becoming a minority with a distinctive culture and set of world views united against a world dominated by heathens and heretics, rather than an establishment universal membership institution. There will be "marketing pressure" for residual Christian communities such as non-denominational churches to downplay the anti-scientific, homophobic, sexist and anti-government stances that helped prompt tens of millions of deconversions. But, paradoxically, the people who remain and organize residual Christian institutions will be precisely those people who weren't turned off from Christianity as a result of those messages.
Apologists who argue that some of the ugly stereotypes about Evangelical Christian or conservative Islamic or ultra-Orthodox Jews aren't accurate in sourcing their unpopular views in scriptural and authoritative religious doctrinal statements are mostly wrong. Saint Paul, the Quaran, and the Torah really do say the things that are totally unacceptable from the perspective of modern Western cultural norms. The "moderates" include mainline Christian churches, to a greater or lesser extent the Roman Catholic church depending on the issue, Reformed and Conservative Jews, the more moderate traditional varieties of Islamic religious practice like the Alawites of Syria, the Sufis of Southern Pakistan, and many of the Muslim communities of Southeast Asia. Each of these religious communities have tamed the ugly core of the religious tradition from which they evolved. Each learned to utilized somewhat tortured interpretations of religious scriptures and doctrines that have become traditional, a pattern of ignoring or deemphasizing inconvenient truths about their religion's doctrines, and mediation of religious instruction through formally trained specialist clergy who have learned to navigate these paths of moderation.
For example, Islamic fundamentalism is more a product not of ignorance per se, but of rising literacy, which allowed ordinary people to bypass the interpretive gloss of traditional Islamic religious figures that used to mediate the messages of the Quaran and other authoritative religious statements like Hadith, to reach the hard core, rather than simply being an elite clergy driven trend. Islamic fundamentalist leaders are people who got out in front of a parade that rising literacy had already brought about whose literalistic and unmediated or interpreted readings of these texts puts them in line with emerging movements driven by the force of ideas in the readings of these core materials without much of a gloss, even when they may take views that are minority ones among formally trained traditional Islamic religious figures (except in a few places like Saudi Arabia which is the hub of the Islamic world as a result of the commandment that Muslims make a haj that have actively coopted the global power associated with fundamentalist movement brought about by global improvements in literacy (but not sophisticated high level interpretive graduate school level moderating concepts) for soft political power reasons).
Indeed, Islamic fundamentalism has a lot in common with the highly unnuanced conservative take on economics that incorporates the overly idealized free market concepts taught in Econ 101 classes in high schools or entry level college surveys, but dispenses with the nuances and qualifications that more advanced classes add to that foundation.
Most non-religious people in the United States are not considered atheists committed to a metaphysically natural worldview philosophically. But, they are also much more secular in their attitudes and beliefs on a whole host of subjects than religiously affiliated theists. There is a great deal of room for an innovative religious movement to emerge and draw in both non-religious identifiers and affiliated but weakly committed Christians and Jews. Although, I doubt that this population is very amenable to inclusion in a formal organization, as opposed to a decentralized movement or trend. But, it isn't obvious to me what direction that might take and if it does take it may look more like superstition or folk religion than it does like organized Christian or Jewish denominationally organized religious bodies.