Measured by religious affiliation, yes, the millennial generation is the most secular in modern American history. Measured by religious attendance, they are the least churched of American adults. That much of the “secular young people” story is true.
But religious attendance ebbs and then flows across the life cycle, falling when you leave home and then increasing with child rearing and with the encroachment of mortality. And when the political scientist Ryan Burge recently compared weekly church attendance among today’s 20-somethings to weekly attendance among 20-somethings in the 1990s, he actually found a tiny increase: Church attendance has been falling among the middle-aged and early-elderly cohorts, but the typical millennial or Gen Z American is slightly more likely to be a weekly churchgoer than a Generation-Xer circa 1995.
So any recent secularization may reflect the aging-and-dying of the more pious Silent Generation, and their replacement in the 60- to 70-year-old cohort by boomers, as much as it reflects the sudden de-Christianization of the young. In which case the “shock” of de-Christianization in the 1960s and 1970s, the years of boomer young adulthood, is arguably still more important to our present situation than the millennial “aftershock.” Boomers kept identifying with the churches they had drifted from, while their millennial children dropped the residual identification. But it was the boomer embrace of religious individualism that really determined the country’s spiritual trajectory, not some unique millennial apostasy.