You go hear even a semi-famous Catholic speaker. More often than not the event is free, the auditorium is half full, and almost everybody’s old.

But last week in a Boston suburb, the Chevalier Theater, 2,036 seats, packed ’em in. Downtown Medford’s parking lots overflowed. Cops directed traffic. It cost $35 a ticket and I’d estimate the average age was 30.

The speakers? Richard Dawkins and Daniel Dennett, two prominent proselytizers for the so-called “New Atheism.”

I guess you could call the evening evidence of what the Pew Research Center on Religion and Society reported last month in its latest faith survey. Christianity, including Catholicism, is tanking in America. The number of non-believers is rising. Nearly 1 in 5 adults identify with no religion at all. And the biggest non-believers are young men and women like the ones who cheered and laughed and had a great old time at the Chevalier hearing two gray-haired guys go on about the ridiculousness of believing something based on nothing.

Let me say right now: As a Catholic who ricochets wildly between blissful moments of faith and complete and utter doubt, I found the whole experience unnerving. I’d hoped both men would be humorless, strident, militant, even obnoxious. Then I could go home feeling confident in my faith. Instead they were funny, charming, and quite likable. I went home deflated. I looked up everyone who’s debated them or contradicted them and argued for the wondrous mysteries of the divine. Then I feel asleep in a funk.

Perhaps you’ve heard of the intellectuals nicknamed the “Four Horsemen of the Non-Apocalypse.” Dawkins and Dennett are two of them. The third was the late Christopher Hitchens, author of “God is not Great: How Religion Poisons Everything, ” and essays proclaiming his non-conversion even as he lay dying of cancer. The fourth is Sam Harris, author of the best sellers “The End of Faith” and “Waking Up: A Guide to Spirituality Without Religion.”

Forget the $35 Dawkins and Dennett charged in Medford. Harris has charged $219 a ticket to detail his no-God approach to spirituality.

Dawkins, an evolutionary biologist and former Oxford professor, is best known for his 2006 blockbuster, “The God Delusion,” which sold more than 2 million copies and was translated into 31 languages. Dennett, a renowned Tufts professor of philosophy, wrote “Breaking the Spell: Religions as a Natural Phenomenon.” He’s also involved in “The Clergy Project.” That’s an online community for religious — priests, nuns, ministers, rabbis, any religious leader — who have lost faith or, as the website puts it, no longer hold “to supernatural beliefs” and face an obvious dilemma: Do they come clean and tell the congregation that their leader, unfortunately, is an atheist now?

The basic premise of the “New Atheism,” a movement about 15 years old, is this: Morality need not be linked to religion and atheists can be as loyal, true, upstanding, and selfless as the saints themselves. Meanwhile, religion, particularly fundamentalism, can be dangerous and should be aggressively opposed when it influences governments, education, and science, impedes social progress, incites violence, or tries to impose its tenets on others. “If you really really believe your God or Allah or whatever wants you to go and do something, “ said Dawkins, “then it’s entirely possible for an entirely rational person to do hideous things.”

Dennett has compared faith to “something that grips the mind in the same way a conventional virus grips the body.” He said it’s like believing in Santa Claus, a myth of childhood, and not something a non-delusional, sane adult could embrace.

Said Dawkins: When we see butterflies, flowers, spectacular sunsets, and other wonders of nature — and assume an omniscient creator at work — remember, Darwin’s evolutionary theory has now shown us otherwise. Today science, not superstition, can answer nearly all the deep questions about why we’re here and the meaning of it all “more grandly and more beautifully” than any religions text.

And the New Atheists hope to educate the uneducated to share this view.

“The word atheist has a kind of horror attached to it,” said Dawkins, adding that the gay rights movement could be a model for non-believers. Suppose all the atheists came out of the closet and made “little YouTube selfies saying, ‘I’m a postman, a school teacher, a nurse. I’m an atheist … and a nice person.’ ”

“When will we have an atheist president? I think we’ve had lots of atheist presidents,” said Dennett. But American politicians must pretend they’re religious to inspire trust. That’s true in regular life, said Dennett, an atheist since his teen years. “But I never talked about it. Then I realized if you actually mention it, no proselytizing, no standing on street corner, you just let it drop (into the conversation), sometimes a person’s face lights up and they say, ‘Oh, I’m not alone.’ ”

In an interview after the Chevalier event, I asked Dawkins if he’s sure he’s right about all this. Is there room for doubt?

“Well, there may be room for doubt in the sense of not understanding. There’s a lot we don’t yet understand.” But that does not mean, he said, that we may one day understand something at all related to the “supernatural.”

In other words, this is it. Here and now. Get used to it.

And Dennett and Dawkins’ Medford fans seemed positively gleeful about it all.

Maybe they’ll rethink when they’re older and facing their own mortality. Alas, Dennett is 73. Dawkins? He’s 74.

There was an hour-long, book-signing line after last week’s lecture. So I asked some of those waiting patiently about the New Atheism’s appeal. Some common answers: It’s much more rational than angels and God-made-man. Too much of organized religion is corrupted by money and power. And cradle Catholics said just what you’d expect: They are unable to remain in a Church that treats women and gays as less-thans and sex as evil. Pope Francis may be a nice guy — and on the right side of climate change — but it’s not enough.

Like I said, it was a tough night.