The depth of her doubts could be viewed by nonbelievers and skeptics as more evidence of the emptiness of religious belief. But Roman Catholic scholars and supporters of the woman who toiled in Calcutta's slums and called herself "a pencil in God's hand" argue that her struggles make her more accessible and her work all the more remarkable.
"It shows that she wasn't a plaster-of-Paris saint who never had a doubt about God or the ultimate meaning of life," said the Rev. Richard McBrien, a University of Notre Dame theology professor and author of "Lives of the Saints." "This can only enhance her reputation as a saintly person with people who aren't easily impressed with pious stories. Those who think otherwise have a lot of learning to do about the complexities of life and about the nature of faith."
This revelation about Mother Teresa's dark years of the soul is not new. Her ordeal, laid out to a series of confessors and confidants, became public knowledge in 2003 during the investigation into her cause for sainthood, a process fast-tracked by Pope John Paul II.
But "Come Be My Light: The Private Writings of the 'Saint of Calcutta,'" to be released Sept. 4 by Doubleday, collects her thoughts in one place for the first time, inviting a closer review of her life 10 years after her death.
The book was edited by the Rev. Brian Kolodiejchuk, a priest who knew Mother Teresa for 20 years and is the postulator for her sainthood cause. It depicts Mother Teresa as a mystic who experienced visions of Jesus speaking to her early in her ministry, only to lose that connection and long for it like an unrequited love for most of her last four decades.
"I have no Faith - I dare not utter the words & thoughts that crowd in my heart - & make me suffer untold agony," she wrote in an undated letter.
In 1956, she wrote: "Such deep longing for God and ... repulsed empty no faith no love no zeal. ... Heaven means nothing pray for me please that I keep smiling at Him in spite of everything."
Mother Teresa acknowledged the apparent contradiction with per public persona, describing her ever-present smile as "a mask" or "a cloak that covers everything."
Some writings seem to suggest she doubted God's existence. She wrote in 1959: "What do I labour for? If there be no God - there can be no soul - if there is no Soul then Jesus You also are not true."
In an interview, Kolodiejchuk argued that, read in context, Mother Teresa's faith remained. Her unwavering belief that God was working through her shows that while Mother Teresa lamented missing that feeling of connection with God, she didn't doubt his existence, he said.
"There's always a risk in publishing like this that some people will misinterpret it," Kolodiejchuk said. "But the far greater good will be for those consoled and encouraged by Mother and her example."
Many other saints and revered religious figures have experienced doubt and struggle; Mother Teresa's namesake, St. Therese of Lisieux, described a "night of nothingness."
What makes Mother Teresa's journey so striking, Kolodiejchuk said, are the depths of her pain, the extraordinary length of it and its documentation in the letters she left behind.
The Rev. James Martin of the Jesuit magazine America, author of "My Life With the Saints," said the window into Mother Teresa's inner life will help doubters and spiritual seekers.
"Most of us tend to think of the saints as being in constant union with God, therefore everything they do is easier for them because of this union. This shows that not only do they have it as tough as we do, but sometimes they have it tougher," he said.
"Other saints did these wonderful things and works for God and on behalf of the church, but most of them had the benefit of a very rich and rewarding prayer life. Mother Teresa in a sense was going on an empty tank all day."
For several Americans who forged friendships with Mother Teresa, her inner torment came as a shock, then started to make sense.
"She had an expression ... 'Give God permission to use you without consulting you,'" said Jeanette Petrie, who co-produced two films about Mother Teresa and traveled extensively with her. "I think she must have truly lived that."
Jim Towey, Mother Teresa's legal counsel from 1985 until her death and former head of the White House Office of Faith-Based & Community Initiatives, said he always presumed Mother Teresa's hardships were physical and that her prayer life sustained her.
Now, he sees suffering consistent with lives of other saints and that of Jesus, who according to Scripture cried out to God on the cross, "Why have you forsaken me?"
"Skeptics are going to say what they want," said Towey, president of St. Vincent College in Latrobe, Pennsylvania, which is hosting a reunion of Mother Teresa's closest friends, relatives and colleagues in October. "But the reality is, any person with faith has doubts."
Ultimately, Towey said he believes Mother Teresa's struggles will demonstrate "faith isn't about feeling, it's about will."
Mother Teresa was beatified in 2003. Under Catholic tradition, an additional miracle attributable to her must be verified for her to be elevated to sainthood.
"This is not any speed bump on her path to canonization," said the Rev. James Langford, who co-founded the Missionaries of Charity Fathers with Mother Teresa in 1984.
"Just the opposite. This is right in line."