From Allen Downey at Scientific American:
Since 1990, the fraction of Americans with no religious affiliation has nearly tripled, from about 8 percent to 22 percent. Over the next 20 years, this trend will accelerate: by 2020, there will be more of these “Nones” than Catholics, and by 2035, they will outnumber Protestants. More.
As Pew notes (2016), however, it’s partly a matter of labelling:
Indeed, our Religious Landscape Study finds a clear generational pattern: Young people who are not particularly religious seem to be much more comfortable identifying as “nones” than are older people who display a similar level of religious observance. Nearly eight-in-ten Millennials with low levels of religious commitment describe themselves as atheists, agnostics or “nothing in particular.” By contrast, just 54% of Americans in the Silent and Greatest generations who have low levels of religious commitment say they are unaffiliated; 45% claim a religion. A similarly striking gap between Millennials and others is also seen among those with a “medium” level of religious commitment.
Put another way, irreligious seniors might describe themselves as Presbyterians, as a mere social distinction. But a Millennial probably won’t do that because today the label implies that one cares somehow. Needless to say, in the current environment, one should use such data with caution when predicting stances on social or political issues.
We actually don’t know that the trend will accelerate* but the real news seems to be buried here:
Millennials are ditching traditional religion for witchcraft and astrology:
Interest in spirituality has been booming in recent years while interest in religion plummets, especially among millennials. The majority of Americans now believe it is not necessary to believe in God to have good morals, a study from Pew Research Center released Wednesday found. The percentage of people between the ages of 18 and 29 who “never doubt existence of God” fell from 81% in 2007 to 67% in 2012.
Meanwhile, more than half of young adults in the U.S. believe astrology is a science. compared to less than 8% of the Chinese public. The psychic services industry — which includes astrology, aura reading, mediumship, tarot-card reading and palmistry, among other metaphysical services — grew 2% between 2011 and 2016. It is now worth $2 billion annually, according to industry analysis firm IBIS World.
Naturalism probably plays a role because, while clearly untrue, it does not point in any direction today except post-modernism (whatever you evolved to believe is the ultimate Cool for you).
See also: Nature, as defined today, cannot be all there is. Science demonstrates that.
and
* Extra! Extra! A handy guide to the normal fake news: Surviving information overload (3. Extrapolations of trends into infinity in a finite world)