My favorite Flannery O’Connor story has a self-satisfied woman sitting in a doctor’s waiting room spewing out a stream of prejudice, when she’s suddenly clobbered on the head with a book.

It was hurled at her by a girl across the room who adds insult to injury by calling the startled woman, Mrs. Turpin, a “wart hog from hell.”

It’s no accident that the girl is named Mary Grace because the story — “Revelation” — is about the moments in life when God sends us a message.

His messages often come through something mysterious and hard to pinpoint with reason, something non-believers don’t talk about. It is called grace.

As a Catholic, O’Connor believed that grace flowed from the sacraments. But she also knew that sometimes grace can strike us in the down-home moments of life, when we’re driving down a road or peeling potatoes or, in the case of Mrs. Turpin, running her mouth in the waiting room.

When I look back on my life, I recall many moments of grace. They didn’t come with angelic hosts singing or with startling sunbursts in the sky.

One moment occurred when my husband returned from a business trip and mentioned he’d lit votive candles in memory of my parents in St. Patrick’s Cathedral in New York City. We were not Christian at the time and his casual statement hit me hard, perhaps like a metaphorical book thrown across the room.

You see, I realized I had never lit a single candle for them myself.

We could have shrugged off that moment and gone on our merry way. Instead, that pinpoint of grace spurred us to go to Mass, which led to my husband’s eventual conversion and my return to my childhood faith.

Later, grace flowed into my life in an equally surprising fashion. A doctor — a nonbeliever — looked at my X-rays after my cancer diagnosis and exclaimed, “It’s a miracle anyone could see this!”

That particular instant of grace sustained me through the grueling months of hand-wringing that followed. And it inspired me eventually to write a book about my journey.

In O’Connor’s story, Mrs. Turpin doesn’t ignore the shocking message in the waiting room.

At home later, she shakes her fist at God, asking how it’s possible for a good woman, as she thinks of herself, also to be a wart hog from hell.

She has a vision in which all the people she looked down on, including the black farm workers and those she called “white trash,” are parading into heaven ahead of her. In a flash, she realizes she isn’t as virtuous as she thought.

We have to keep our eyes and ears attuned to receive grace. It may come quietly, in a conversation with a friend or with a dramatic knock on the head.

Grace is a gift and freely given. We can either accept it and take it to heart — or walk away.