Three priests ― a Dominican, a Franciscan and a Jesuit ― walk into a bar.

According to the Rev. James Martin, it’s not only the opening to a good joke, but quite possibly the saving grace of religion.

Martin's new book, Between Heaven and Mirth: Why Joy, Humor, and Laughter Are at the Heart of the Spiritual Life, says religious people would be a lot happier ― and holier ― if they lightened up and took themselves a little less seriously.

“Joy, as a number of spiritual writers have said, is the surest sign of the Holy Spirit,” the Jesuit priest said at a recent gig at Georgetown University.

But, he continued, “there are certain Roman Catholics who seem to think that being religious means being deadly serious all the time.”

Martin, culture editor of the Jesuit magazine America and the unofficial chaplain to Comedy Central’s “Colbert Report,” is, well, wickedly funny.

Martin conceived the idea for his book while promoting his earlier book, My Life With the Saints. He found that audiences seemed most interested in stories that showed the saints to be full of joy and good humor. Or, in other words, a little less pedestal, a little more human.

“I think Catholics are so used to hearing about the saints as kind of dour, gloomy, depressed people, that that was a surprise to them,” Martin said. “It was almost as if Catholics were hungry for that permission, almost, to enjoy themselves.”

The other thing he found was the irony of working with “professional” religious people who were, “in a word, grim.”

While joy shows a person is growing closer to God, Martin argues, it is also crucial to the health of the church. “Humor evangelizes,” he said simply.

Addressing an audience at the U.S. Conference of Catholic Bishops recently, he added that the best way to increase vocations to the priesthood or the convent is to “live your own vocation joyfully. Why would anyone want to join a group of miserable people?”

Although Martin uses his own Catholic faith most frequently in his examples, his argument extends to other faiths, he said.

“Stories from a variety of religious traditions show that humor can serve some serious purposes,” he writes, pointing to funny Talmudic legends, Confucian aphorisms and Zen Buddhist koans.

Martin said humor ― especially self-deprecating humor ― is “essential” for interfaith dialogue because it “relaxes” the discussion.

“It humanizes you, and it also reminds people that ... you take your faith seriously, but you don’t take yourself too seriously,” he said.

Of course, Martin admitted, there are certain lines that cannot be crossed when using humor around religious matters. He’d never tell a joke “making fun of certain people, making fun of certain Catholic beliefs, denigrating people, anything that’s ad hominem.”

“The best jokes are the ones that poke fun at yourself. So I can tell jokes about Catholics, Jesuits and priests, because I’m all three,” he said. His favorite thing to poke fun at when it comes to Jesuits is their perceived worldliness, at least compared to other religious orders.

Martin said he “would never and could never” make a joke out of something like the sexual abuse scandals in the church. Humor about the pope is OK, he said, especially if the pope invites it.

“Even better are jokes that the popes themselves tell,” he said, referring to one time when Pope John XXIII made fun of an archbishop’s growing girth. Asked once how many people worked in the Vatican, John XXIII replied, “about half.”

Efforts to inject merriment into matters of the spirit are not always met warmly. Martin said he “quite frequently” gets into trouble with his jokes.

“If you ever tell a joke that includes Jesus or Mary, not as the object of fun, but even if they’re in the joke ... some people don’t like that. People should know the difference between humor and disrespect,” he said.

Currently, Martin is working on a book about the life of Christ. “The definitive book on Jesus,” he said, laughing.

Then he added, more soberly, “Make sure you say, ‘He said jokingly.’ You never know, with religious people. That’s part of the problem. Even something that is clearly a joke ... you can see people taking it seriously and being offended.”