Once I believed that when you found faith, it rarely wavered. Then I learned that even saints had massive doubts about God.

How reassuring.

If even the holiest of the holy had second thoughts, why not me?

Maybe we Catholics should talk more about doubt.

It actually is an intrinsic part of the pilgrimage, a Jesuit friend priest told me, common at the beginning and throughout the spiritual journey. Then he told me to read none other than former Pope Benedict XVI on doubt.

Indeed, the first chapter of Joseph Ratzinger’s “Introduction to Christianity” is all about doubt vs. belief. “The believer is always threatened with an uncertainty that in moments of temptation can suddenly and unexpectedly cast a piercing light on the fragility of the whole,” he writes. Suddenly the believer is not just questioning the literalness of biblical stories — whether, say, Christ really walked on water — but facing “the bottomless abyss of nothingness.”

And the abyss is lurking everywhere, it turns out.

Saint Therese of Lisieux, a 19th-century French Carmelite nun, wrote about her own terrible crisis of faith at the end of her life, at a mere 24. The nuns she lived with were so horrified they edited her writings to remove mentions of the “temptations of atheism.”

Spiritual genius Thomas Merton, the famed Catholic monk, said in “New Seeds of Contemplation,” “Let no one hope to find in contemplation an escape from conflict, from anguish or from doubt . . . for every gain in deep certitude there is a corresponding growth of superficial doubt.”

Some of the best-known Catholics novelists of the 20th century — Graham Greene, Flannery O’Connor, Walker Percy, Mary Gordon — created characters that swing wildly between faith and doubt. A recurring theme: Faith is so hard to maintain in a brutal, unjust world; doubt comes easily.

Most famously and recently, Mother Teresa of Calcutta, whose letters were released in 2007, expressed doubt and despair about God. Her “dark night” lasted almost 50 years, with rare reprieves, up until her death in 1997.

Meanwhile, juxtaposed to all this uncertainty are the doubters who insist with total certainty that there is no God, period. (This group would include the so-called New Atheists like writers Sam Harris and the late Christopher Hitchens). Conversely, the fundamentalists and many evangelical Christians insist with equal certainty that every word of the Bible is 100 percent true and literal, period.

“We have developed a kind of lust for unsustainable certainty.” So said Karen Armstrong, a former nun and author of more than 20 books, including “The Case for God,” in a 2010 interview with US Catholic.

She compares such certainty about God to the “infantile” certainty small children feel about Santa Claus. Obviously our ideas about Santa Claus evolve. Yet many of us hold onto the same childlike vision of God at 40 that we had as seven-year-olds heading off to First Communion: some bearded granddaddy in the sky.

God cannot be proved, Armstrong says. “In his Summa Theologica, (St. Thomas) Aquinas starts out by saying that we cannot define God. Then he gives us five ways, as he calls them, to think about God,” she told US Catholic. “He ends each way by saying this is what everybody means when they say God. Then he immediately pulls the rug out from under our feet and says we have no idea what such a being is or how it can exist. We can’t even say it exists. All we’ve proved is the existence of a mystery.”

Oddly enough, in “Introduction to Christianity,” former Pope Benedict himself also acknowledges this mystery and accepts uncertainty and wavering between faith and doubt as an inescapable part of “the dilemma of being a man.” Nobody can be certain, not the believer or the unbeliever either.

Again, how reassuring, to those with fragile faith and those who lost faith entirely and yearn to get it back?

“Just as the believer knows himself to be constantly threatened by unbelief,” Benedict writes, “for the unbeliever faith remains a temptation and a threat to his apparently permanently closed world.”

Then Benedict cites a wonderful story by the Jewish philosopher Martin Buber. A scholar of the Enlightenment enjoyed debating rabbis in hopes of shattering their proofs of God’s existence. One day the scholar goes to debate a particularly famed Rabbi. But before he can begin, the rabbi turns to the doubting scholar and says, “My son, the great scholars of the Torah with whom you have argued wasted their words on you; as you departed you laughed at them. They were unable to lay God and his Kingdom on the table before you; and neither can I. But think, my son, perhaps it is true.”

And that captures perfectly the situation of a human being confronted with the question of God, Benedict writes. No one can lay God and his kingdom before another. But however certain the unbeliever, however doubting the believer, neither can “forget the eerie feeling induced by the words, ‘yet perhaps it is true.’ ”