The Christian pews have gray hairs but not purple streaks. The Mosques have mothers but few daughters. The synagogues have Priuses parked out front, but not skateboards.

What’s changed? American millennials are much more likely to say they don't identify with any religion. Three times more likely than their great grandparents, in fact, according to a Pew Research Poll.

Millennials are the least religious generation in history, and getting less religious every year. What's kept quieter, though, is that millennials still believe in something at almost the exact same rate as their great-grandparents … just not the same something they did. The same high percentages of both millennials and the Greatest Generation think about the meaning and purpose of life, feel a sense of gratitude, and feel a sense of wonder about the universe.

The question is: if millennials aren't religious anymore, to whom are they grateful? Why do they feel a sense of wonder? What do they consider the meaning and purpose of life?

After spending some time thinking about it, reading about it and asking around, I've learned there are no easy answers. So maybe it's easier to start with what millennials don't believe in.

They don't believe in the One True Faith. Millennials will never believe, as their grandparents did, in God and Jesus and the Holy Spirit, in church on Sundays and grace before meals, in a church tower in the town square and swearing on the Bible in court.

Even within the millennial generation, the old Christian rituals are dying like flowers in the desert. Younger millennials, aged 18 to 24, pray less, read scripture less and think the bible is the word of god way less, than older millennials, aged 25 to 33. God is, to most people reading this, not a man in the sky. Nor is he a man who spent some time nailed to a cross before crawling out of a cave.

So, if not Jesus, then … what?

It seems that millennials find their deepest truths in a crazy hodgepodge of things: science, music, nature. These are things you don't typically think of as religious — but they do, like religion, have rituals and order and history, and they are endlessly deep and rewarding. There's no limit to the faith a person can have in science or music or nature, in the way medieval peasants had faith in a single god.

In fact, "music is replacing religion," according to Clive Marsh, a British professor who studies both. People use music to explore the philosophical and ethical issues of the modern world, he told the Telegraph.

Take a look: nothing resembles an old religious revival like a concert. Bassnectar, Insane Clown Posse and Bieber fans can be as devoted to their rock gods as Hindus marching to the shrine of Shiva.

"The Grateful Dead is pretty much a religion," says devoted Deadhead Sasha Bellucci of Boulder. The Deadheads tattoo themselves in the symbolism of the dancing bears and the Bertha skeletons and treat Jerry Garcia like a martyr who died for the music. Like religion with its wine, music has its holy sacraments: marijuana at reggae, molly at Pretty Lights. And what is following a band but the modern-day version of a Catholic pilgrim on the road to Compostella?

Science is another popular alternative route to finding transcendence. Along with being elegant and egalitarian and letting you wear a sparkling white coat, the things science reveals stand a high likelihood of being true. And what truths! The universe was once smaller than the tip of a pen! The material in your body was cooked in the center of a star! There are probably aliens — real beings up in the real sky we might one day really talk to!

But what about morality? While science didn't used to touch on questions of right and wrong, people are slowly getting more guidance here. That increases as you go younger, with even more younger millennials getting guidance about what's right and wrong from science and reason than older millennials do.

Nature is tied up in all of this, too; scientists study nature, and some of the best concerts happen outdoors at Telluride or Red Rocks or The Gorge Amphitheater. In Western cities, millennials don't spend their Sundays in church — they spend them REI. They don't visit towering steeples, they visit towering fourteeners. The mark of devotion isn't ash on the forehead, it's mosquito bites and chain rash. Some people find God in the woods rather than in a church, Michael Hout, a professor of sociology at New York University, told Pew Research.

In fact, defenders of the old Jesus religions can be critical of the new church of the outdoors. They say millennials treat the natural world as a god itself, and Global Warming is our great sin against it. "Thus has the God of Abraham been replaced by the God of Atmosphere;" scoffs the Christian-leaning website The Federalist. "Salvation by Christ is increasingly being supplanted with salvation by carbon tax."

That may be. But as old institutions fall away, this generation has to search for meaning. Luckily for them, younger millennials were born with the Google search bar waiting. So it was impossible to grow up without knowing that there were other gods besides your god, other rules besides your rules, other funny hats besides the funny hats your priests wore. And there was always plenty of other options about what you could follow, where you could find meaning, so you could decide: "I don't want to be a Lutheran, I want to worship K-pop."

Don't be misled: god ain't dead. Most people — even younger millennials — still say "they believe in god," Pew says. But the old god is having a tough time lately. Younger millennials are 50 percent more likely than older millennials not to believe in him.

But millennials still believe things. Remarkably, they still believe in the immortal soul. Yes, almost the same percentage of younger millennials believe in an afterlife as their grandparents and great grandparents did.

What god, exactly, they'll meet in the afterlife is an open question — possibly even to them.