As I noted in a post earlier today, secular Americans are most common in the West, then the Northeast, then the Midwest, and then the South. More whites are secular than blacks and Hispanics, although Asian-Americans are more secular.
One of the leading secular organizations in the United States, the American Ethical Union, has its roots in Jews who lived in or derived from New York City Jews. Another, the American Humanist Association, is basically an offshoot of the Unitarian-Universalist movement, two denominations that have their roots in Boston. The Unitarians were derived from the Congregational church which was derived from the establishment Puritan church of New England (the Trinitarian offshoot of the Congregation Church was the United Church of Christ). Both offshoots emerged as independent denominations in the early 1800s when the Congregational church was disestablished. The Universalists were a working class white storefront church movement that began at about the same time in Boston. New England Transcendentalists and Revolutionary era deists (mostly, but not entirely from the Northeast) were also important in the legacy of secular America.
Bertrand Russell, the British philosopher whose most important works (e.g. "Why I am not a Christian.") were published in the early 20th century were part of the New York City intellectual scene when he found an appointment as a professor of philosophy at the City University of New York after being discharged from his post at Cambridge and jailed for six months by being an active British Pacifist during World War I. He would in due course be discharged from his CUNY post in the 1940s in the face of complaints about his undue openness in a philosophical treatment of marital sexuality, his atheism and his pacifism. Sometime after that point, he returned to his native England, although he would face arrest and be jailed again for anti-war protests as an elderly man.
All of this is somewhat ancient history, however. The percentage of Americans who identified as not religious was pretty much constant until around 1990, after which it has steadily increased. The study that produced an academic understanding of the historical roots of the modern secular movement in the United States took place mostly after the people driving that movement were aware of it and had made their cultural and religious choices in their guts.
Unitarian-Universalism wasn't reformulated as a para-Christian or post-Christian denomination until the 1960s and 1970s, which is also about the time of an important wave of modern East Asian migration, an increasing popularization of Eastern religious ideas, a time when many traditional cultural ideas in the United States and the West were challenged, a time when birth control (a constitutional right to which was established in a Connecticut case) and the feminism more generally upset gender roles, a time when gay rights first emerged (Yankee settled San Francisco and Northeastern New York City were its epicenters), when the Boy Scouts of America began to welcome non-Christians (but not atheists and agnostics) into their ranks, and when advanced in disciplines like physics, genetics, linguistics, paleobotany, historical biblical criticism, and anthropology began to put the nail in the intellectual coffin of a literal reading of the Book of Genesis and the Evangelical Christian world view. More Eastern religious influence followed Southeast Asians who joined American lovers, parents and allies with whom they had made contact during the Vietnam war.
American secularism surged not then, but a generation later, when the children born once this cultural revolution had really embedded itself in their entire lives (and their parents' and teachers' attitudes about raising them), reached adulthood.
The rise in secularism was also was a reaction to the emergence in the 1980s of the conservative, anti-intellectual, and mostly Southern and rural Evangelical Christian movement as a political force and the face of Christianity (a movement itself energized by political and cultural resistance to a Yankee driven civil rights movement with roots as old as the abolition movement and after that, the post-Civil War Reconstruction, and further empowered by Eisenhower era, anti-Soviet motivated governmental ceremonial Christian observances), which the more liberal liturgical and Black Protestant Christians who had fought the Civil Rights movement together never really effectively rebutted in the political and public sphere with an alternative image of Christianity.
This Evangelical Christian movement that claimed to speak for Christianity generally without an effective rebuttal, espoused a set of values and an approach to understanding what the Bible commanded of Christians that was utterly foreign to mainline white Christians in New England, the Pacific Northwest, San Francisco, much of the Northern Midwest, and immigrants from Western Europe. These mainline Christians had been raised to believe in a tolerant, social gospel Christianity far removed from the de-emphasized pronouncements of Saint Paul's epistles and Old Testament rules designed for the culture of honor of Jewish herders and warrior-conquerors who had ceased to exist around the time of the Jewish diaspora a thousand years after their laws were committed to writing. Examining the claims made by Evangelical Christians made young Northern Christians aware of parts of the Christian tradition and doctrine they would never have been aware of and hence, never repulsed by, otherwise. Their local clergy didn't make these points, but weren't very effective in forcefully disavowing them and condemning Evangelical Christians as not very Christian themselves either.
While the churches that they were raised in supported the values that they held, they did not need churches to find ways to live the values that they were taught in church growing up in a nation where tolerance and the social gospel were the values of the secure establishment culture and a part of the secular law. In contrast, Evangelical Christianity preserved the culture of the American South which was in the wake of the Civil Rights movement's successes more threatened than it has been since Reconstruction, while Roman Catholicism was also a haven for preserving old world culture for many Hispanic immigrants, and Black Protestant churches were a haven in which a black culture that had never been an elite culture in the larger world almost anywhere sustained itself. The young people who have become America's non-religious people were mostly drawn from the ranks of people who didn't need religion to provide a haven for their values and cultural ideas because their values and culture were the elite and dominant culture of the United States already. Also, their parents had already disrupted the one powerful role that mainline Christianity had continued to provide in the North until then, the notion that the church was central in the script of the life cycle of birth, coming of age, marriage and death. Their counterculture biographies, the generation gap between them and their parents, feminism, and the divorce culture that emerged at this point in time, all proved otherwise and undermined the cultural authority of the church for mainline white Christians in the process.
Science was increasingly making it possible to have a worldview that didn't need religion to fill the gaps, so a search for a new religious home after young adults rejected the Christianity that they were born into wasn't such an urgent task. Their affluent upbringing compared to their ancestors also removed from their lives much of the uncertainty that drives a psychological need to call upon metaphysical favor. These were children raised to believe in meritocracy and the security of the social safety net.
This generation also had the example of the dramatic secular shift of Western Europe, and increasing awareness of a more secular approach to an increasingly modern life in Asia to look to as well. The most secular places in America today already had an instinctual opposition to Dixie cultural initiatives. These regions saw large waves on immigration from less Christian Europe and and Asia into many professional caste positions in the 1960s and 1970s and beyond. In the case of the Northeast, an existing history of integration of Jews into the ranks of the previously WASP elites in the 1960s, in addition to non-Christian or only nominally Christian immigrants and the need to accommodate the religious diversity that had long existed there, created elite tolerance in both the Northeast and the Pacific Coastal states for a critical mass of non-Christians and heterodox Christians in daily elite life.
Meanwhile, at this point in time, Evangelical Christianity had come close to becoming an unofficially established religion, with only very modest mainline Christian minorities often themselves denominationally divided on North-South lines, and very few Catholics or Jews or Lutheran or Mormons or Orthodox Christians in much of the South. In the South, it was only by the 1970s and 1980s that Northerners, a large share of whom were part of economic elites, began to migrate into the South's economically developing urban centers in large numbers, creating communities of white and Asian "carpet baggers" who did not share a sense of cultural solidarity with the native Southern whites who preceded them. Others, inspired by the Civil Rights movement, moved South precisely to reform the South culturally, taking up a cause that had been abandoned when New England school teachers had been dispatched back to the North by local schools after Reconstruction ended.
Some of the migrants assimilated, but many who moved to the South with no desire to become a part of Southern culture, did not. They were lured by the power to make good money transforming the South's economy into one consistent with their outsider's vision in a place that the Civil Rights movement had made more culturally tolerable, or as cultural missionaries seeking to reform the South's culture, not by a desire to join the ranks of likeminded people and share their historic vision of civilization themselves.
Cheaper and more widely available long distance telephone service, Hollywood and New York centered mass media, cheap long distance travel on commercial jet airplanes, Civil Rights laws suppression of local political power in the South, Warren Court era suppression of government limitations on freedom of conscience with roots deep in William Penn's colonial America, and eventually, the Internet, dramatically limited the social and legal pressure that Southern communities could impose on migrants who didn't want to assimilate local cultural norms. So, some cultural foundation for a smaller but more motivated secular minority was put in place in the South as well, and then promptly threatened by an Evangelical Christian movement that had made a move to take political power and restore some measure of established religion on its communities.
Notably, in my own home in Colorado, the secular organizations in the Evangelical Christian hotbed of Colorado Springs with Southern cultural roots, are much better organized and militant, despite having smaller numbers of non-religious people to draw from than in more secular Denver with Yankee cultural roots. There is scarcely any organized secular movement at all in Pueblo, even further to the South, with long standing Southwestern American roots dating back to old Mexico and a tradition of casual and less militant and less political Roman Catholicism.
Why didn't the United States experience secularization when Western Europe did?
1. In most of Europe, an expressly Christian Democratic party embroiled Christianity in politics in the immediate post-war period, mobilizing parties of the left to be openly anti-religious much earlier than in the U.S. where few political forces had so explicitly aligned themselves with religious factions until the 1980s.
2. A history of dominant established religious denominations weakened the need for churches to adapt themselves to win enthusiastic voluntary participation from their members, so when society collapsed after World War II, their religious institutions were less vital. Competition for centuries and an absence of strong governmental support had made American religious organizations institutionally stronger. Moreover, the need of Europe's established religions to remain acceptable to the rulers of the states they relied upon for their established status, prevented the anti-authority, anti-intellectual, fundamentalist Christian that had been endemic in America at least a century by World War II from emerging. England and Europe generally had exported their most hard core religious dissenters to the United States for centuries and kept their own faiths tame and less threatening to establishment values in the process.
3. European religious institutions, by and large, weren't strengthened as a means of protecting immigrant cultural traditions or outside a few notable exceptions like Ireland, of protecting local cultural traditions from elites imposed by outsiders. Europe is mostly not a continent of recent immigrants (or wasn't anyway, during the period of European secularization). In contrast, in the United States, a nation of immigrants that received a major new wave of immigration in connection with World War II, there were always many more relatively young immigrant churches whose cultural preservation role kept them vital.
4. World War II was fought on lines that bridged the historical Protestant-Catholic-Orthodox religious divides of Europe on both side of the conflict rather than being a conflict framed by historic religious boundaries. Catholic French waged war against Catholic Germans. Nazis were both Protestant and Catholic united by nationalism and language despite their division religiously. Much Nazi symbolism while anti-Jewish, was imagined in the tradition of an ahistorical form of Aryan paganism, and the Axis powers included the non-Christian Japanese. Protestant Christians in Britain allied themselves with the secular Soviet Union and identified with the secularist left that lost the Spanish Civil War. As a result, World War II didn't mobilize people on religious lines against foreign religious threats in the same way. Instead, the Nazis were cast as uber-conservatives encouraging the whole of European society to a generally liberal reaction in the immediate aftermath of the war. Even respectable European conservatism was wary of straying into the realm of fascism and wary of the Nazi use of superstition to consolidate power.
5. The more comprehensive social safety net enacted in the post-war period after seeing how its absence had helped fuel the Nazi movement reduced uncertainty in daily lives and the instinct to turn to prayer to deal with uncertainty. Also, almost every place in Europe had some period of anti-clerically inclined democratic socialist rule. In the U.S., in contrast, anti-Soviet sentiment fueled conservative political gains and anti-secular sentiment as the Cold War arrived, and the social safety net remained relatively weak even after the New Deal.
6. World War II utterly interrupted the forces of tradition almost everywhere, while there was no interregenum of disrupted traditional authority in the United States until the Civil Rights movement was successful. The United States held elections as usual in 1942 and 1944, right in the middle of World War II.
7. Very little of the immigration that disrupted cultural patterns in the Northeast and West had parallel levels of immigration to the South or to the rural Great Plains until after the Civil Rights era because the stagnant economy there (employment-wise anyway) didn't need them. Industrialization needed people in cities and the "Great Migration" in U.S. history was from the South to the North to feed the labor needs of factories, not the other way around to feed the labor needs of the more agricultural Southern economy. Thus, traditional religious folkways were not interrupted in the South, while the North saw more disruption and Europe saw its whole society disrupted. Secularism was greater in the North than in the South and remains so, in part due to the impact of this migration. Religion can't be a credible basis of a majority political coalition as it did in the South eventually without a degree of religious homogeneity over large geographic areas that wasn't present in the urban North.