(CNN) Pope Francis invoked four Americans in his historic speech to Congress on Thursday. Two are instantly recognizable: Abraham Lincoln and the Rev. Martin Luther King Jr.

The others -- Dorothy Day and Thomas Merton -- were Roman Catholic activists and ardent pacifists but lesser known, until the papal mentions sent many listeners on a Google search.

Thomas Merton

Merton, a French-born Trappist monk and arguably the most influential Catholic writer of the last century, converted to Catholicism after a stormy start in life. His parents were artists, and both died before Merton grew to adulthood.

In his most famous book, the autobiographical " The Seven Storey Mountain ," Merton describes a wild and indulgent youth that involved excessive drinking and womanizing. He even fathered an illegitimate child.

"I labored to enslave myself in the bonds of my own intolerable disgust," he wrote.

Searching for meaning in his life, Merton boarded a train in December 1941 that took him from New York to the Trappist abbey in Gethsemani, Kentucky , where he immersed himself in the Catholic Church.

Merton was later hailed as the conscience of pacifists. He was a strong supporter of the civil rights movement, which he called "certainly the greatest example of Christian faith in action in the social history of the United States." He and King were very much engaged in each other's work, said Mark Meade, assistant director of the Thomas Merton Center in Louisville.

In the last years of his life, Merton spent time exploring Eastern religions, especially Zen Buddhism. He visited India, met with the Dalai Lama and studied similarities between Buddhism and Christianity. The Dalai Lama, Meade said, praised Merton by saying he had a deeper understanding of Buddhism than any other Christian he'd known.

Merton stirred controversy with his beliefs among Catholics, certainly, but also among Christians at large who saw his social activism as ill-fitting for a monk. People angered by Merton's stance against the Vietnam War burned his books not far from the Kentucky abbey.

"Merton challenged people on the left and the right," Meade said. "It's evident that (the Pope and Merton) care about the same sort of themes. In some ways it makes a lot of sense the Pope mentioned him. But we didn't see it coming."

Merton died in Bangkok in 1968, the victim of an accidental electrocution. Thursday marked a redemption of sorts when the Pope called him a "thinker who challenged certitudes of his time and opened new horizons for souls and for the church."

Dororthy Day

Before Dorothy Day became a devout Catholic -- the sort who's being considered for canonization and was given a shout out by Pope Francis in his address to Congress -- she was, in some people's views, far from saintly.

She was a radical journalist, an activist who was branded a communist and socialist. She was a free-wheeling woman who had affairs. She drank, she smoked and, get this, she had an abortion. Later, she gave birth to a child out of wedlock.

"We know all of that because she told us herself," says Paul Elie, who wrote about Day -- and others, including Merton -- in " The Life You Save May Be Your Own: An American Pilgrimage.

What's made her controversial, while being considered for sainthood, is not these matters but rather her position as a pacifist and anarchist who opposed all war and called out bishops who failed to do the same, says Elie, a senior fellow in Georgetown University's Berkley Center for Religion, Peace & World Affairs . During the Cold War, most notably, she spoke out against American militarism, "in the name of Catholicism," while criticizing bishops who backed America's policies.

"She was a faithful daughter of the church but had no hesitation condemning things she thought were wrong in the Catholic world," says Elie, who is covering the Pope's visit to the U.S. for Vanity Fair.

Day converted to Catholicism in the late 1920s and died in 1980 at age 83. She was the co-founder of the Catholic Worker Movement , a staunchly pacifist movement that works on behalf of the poor and homeless. And it's who she became that Pope Francis chose to honor.

"Her social activism, her passion for justice and for the cause of the oppressed, were inspired by the Gospel, her faith, and the example of saints," the Pope said.