Ten years after her death, a new book of Mother Teresa's personal letters illustrates a profound and private spiritual struggle— much of it unknown to the world that would come to embrace her as a living saint.

Mother Teresa: Come Be My Light, to be released Sept. 4, is a collection of Teresa's personal letters to her spiritual advisers. For the most part, they are letters she never intended to become public and, in fact, had asked to be destroyed.

In one letter from 1962, Teresa even mused about how her sense of spiritual desolation might impact the bid—now under way at the Vatican—to make her a saint.

"If I ever become a Saint—I will surely be one of 'darkness,'" she wrote. "I will continually be absent from Heaven—to (light) the light of those in darkness on earth."

The book will likely challenge the characterization many people had of Teresa as a simple, pious woman, said the Rev. James Martin, a Jesuit priest who wrote the best-selling My Life With the Saints.

"I think that this is a real treasure for not only believers, but even doubters and skeptics," Martin said. "I think it also makes her much more accessible to the everyday believer. It shows that even the saints struggle in their spiritual lives and that they don't have it easier than we do. They sometimes have it harder than we do."

The book was edited by the Rev. Brian Kolodiejchuk, who directs the Mother Teresa Center from Tijuana, Mexico, and oversees her cause for sainthood; she was beatified by the late Pope John Paul II in 2003.

As part of the bid for sainthood, Kolodiejchuk read through 6,000 of Teresa's letters. For the book, he included letters pertaining to three aspects of her life: her vow to God, what she called "the inspiration" and also "the darkness."

In 1942, Mother Teresa made a vow not to refuse Jesus anything. Starting in 1946, she experienced several mystical encounters with Jesus, whom she called "the Voice," asking her to serve "the poorest of the poor." The "darkness" was her term for feelings of loneliness and abandonment when her communion with Jesus ended.

Prior to 1946, Kolodiejchuk said little was known about Teresa's spiritual life. "She says in a letter, 'I came to India with the desire to love Jesus as he has never been loved before,'" he said. "She was a woman passionately in love with Jesus."

Yet no sooner did Teresa start her work in the slums of Calcutta than she began to feel the intense absence of Jesus—a state that lasted until her death, according to her letters.

"The paradox is that for her to be a light, she was to be in darkness," Kolodiejchuk said.

In a letter estimated to be from 1961, Teresa wrote: "Darkness is such that I really do not see—neither with my mind nor with my reason—the place of God in my soul is blank—There is no God in me—when the pain of longing is so great—I just long & long for God. … The torture and pain I can't explain."

Over time, the Rev. Joseph Neuner, a spiritual adviser, helped Teresa realize her feelings of abandonment only increased her understanding of the people she helped, Kolodiejchuk said. Ultimately, she identified her suffering with that of Jesus, which helped her to accept it.

Catholic saints typically experience a "dark night of the soul" in the words of 16th-century priest St. John of the Cross, Martin said, but never as long as the "whole working life" Teresa experienced.

"She moves into the ranks of the greatest saints," Martin said. "There are very few who have suffered such an extended dark night."

But Martin stressed that Teresa's belief in God never wavered—just her feeling of connection to Jesus, especially after her intense mystical experiences.

"It's one thing to feel that God is not with you. It's another thing to believe that God doesn't exist," he said.

How all this may affect her bid for sainthood remains unclear. Some say it makes her even more impressive. While her spiritual loneliness became known during the canonization process, Kolodiejchuk said this is the first time the arc of her inner spiritual life is compiled in one place in her own words.

Gezim Alpion, the author of Mother Teresa: Saint or Celebrity? who teaches at the University of Birmingham in England, wondered whether the Catholic Church was trying to capitalize on Teresa's popularity by releasing her private confessions.

"It's atrocious to think that whatever you tell these people in confidence becomes popular," he said.

Despite Mother Teresa's requests for privacy, publication of her private thoughts should be expected, said the Rev. Richard McBrien, professor of theology at the University of Notre Dame and author of Lives of the Saints.

"Nothing," he said, "is private in a canonization procedure."

Related Elsewhere:

Mother Teresa: Come Be My Light is available from ChristianBooks.com and other retailers.

Time's September 3 cover story is "Mother Teresa's Crisis of Faith."

The New York Times published an op-ed on the significance of the letters.