On December 25, Mark Zuckerberg posted a message to Facebook in which he wished his followers a merry Christmas and happy Hanukkah — a bit surprising since Zuckerberg had previously described himself as a non-believer.

When one commenter asked the Facebook CEO if he was still an atheist, Zuckerberg said no. “I was raised Jewish and then I went through a very important period where I questioned things, but now I believe religion is very important,” he wrote.

Although scholars have studied the effects of converting from one religion to another, almost no comparable research has been done on what makes atheists return to religion.

“It’s sort of an emerging field,” said Alexander Angelov, an assistant professor of religious studies at the College of William and Mary. According to him, atheists returning to religion “is a relatively wide phenomenon, but scholars haven’t caught up to it.”

The rarity of atheists picking up religion may be why it’s so understudied. It’s far more common for religious people to become atheists than it is for non-believers to find God, according to a Pew analysis: Four religious people reject religion for every non-religious person who converts to it.

One of the best looks at this phenomenon is an analysis of a 1978 Gallup poll of “unchurched Americans.” This study found that 46 percent of Americans drop out of active religious participation at some point in their lives, with the number of dropouts from organized religion peaking in the 1960s. The study also found that 80 percent of those who drop out of organized religion end up returning to their religious institution at some point in their lives, although the religious re-entry rate was highest among 25- to 34-year-olds.

It’s far more common for religious people to become atheists than it is for non-believers to find God.

The Gallup study doesn’t explain what drives people to return to their religion after a period of disengagement, however. It would be another 30 years before this question was partly answered in a 2010 study by Duke psychology and neuroscience professor Aaron Kay, who found that government instability results in an increase in religious belief in individuals.

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In the former Soviet Union, all forms of religion (but especially Christianity) were persecuted. When the Soviet Union collapsed in the early ’90s, there was a resurgence of religious belief. Since 1991, the number of Russian adults who identify with Orthodox Christianity rose from 31 percent to 72 percent of the population.

“When you have a crisis of institutions, people seek security and alleviation of their fears elsewhere,” said Angelov, who grew up in the former Soviet Union and witnessed this firsthand. “You see people who had never thought about religion before all of a sudden putting their trust into all kinds of religion because presumably the church is not governed by people — it’s governed by God.”

Age is another factor that correlates with increased religious observance. A 2012 study from the University of Chicago’s National Opinion Research Center, which followed a group of people from over 30 countries for nearly three decades, found that 43 percent of people over age 68 said they were certain that God exists, compared to only 23 percent of people under 27. According to the study, most of this increasing belief in God occurs between the ages of 58 to 67.

At the same time, there is a broader trend in the U.S. toward an ambiguous “spirituality” among 18- to 34-year-olds. In practice, this means an increasing number of millennials are unaffiliated with any particular organized religion but still have a belief in some sort of a higher power.

This likely has to do with the “perceived political entanglement of religion, especially Christianity,” said University of Virginia religious studies professor Matthew Hedstrom, referring to institutional crises like the Catholic sex scandal and the debate about gay marriage.

The spirituality craze may be picking up a few atheists as well, as young people in the U.S. are keeping God but throwing out the church. Hedstrom described the millennial move toward spirituality as “what consumer capitalism does to religion.” According to Hedstrom, an individual’s identity under consumer capitalism is based on consumption patterns. Ultimately this results in an individual picking and choosing which “religious products” (meditation, prayer, yoga, and so on) they want to consume, rather than belonging to an organized religion. Atheists may not be ready to accept a religious text about God and an organized religion, but they may be amenable to this sort of fun, social, more amorphous godliness — especially if it’s trendy.

This also seems to be in line with the general philosophy of Silicon Valley, the front line of technological advancement. Here, the positive benefits of religious practices are increasingly applied in an otherwise secular environment to create a sort of Burning Man-inspired techno-paganism. In the valley, it is not uncommon for tech companies to integrate spiritual practices into their workplaces — Google has embraced meditation, the smart drugs R&D company Nootrobox encourages days of fasting, and Facebook has set up Buddhist-inspired “compassion research days.”

Even with increasing rates of atheism — the percentage of Americans who identified as atheists nearly doubled between 2007 and 2014 from 1.3 percent to 3.1 percent — the U.S. is still heavily religious. Zuckerberg is now in the majority.