In dozens of letters spanning 66 years, Mother Teresa described the "emptiness" she felt and confessed her struggles with faith and the existence of heaven in pages she had planned to have destroyed.
A decade after her death, they have been published in the book "Come By My Light" as part of the petition for her sainthood.
"The lives of the saints are personal, but they are not private," said The Rev. Brian Kolodiejchuk, who is publishing the letters. "The documents are really are quite valuable in that they speak of her own holiness and the value … to people who can relate to what she was going through."
They offer surprising revelations, including one instance in which she writes, "no faith -- no love -- no zeal -- [The saving of] souls holds no attraction -- Heaven means nothing … it has been like this more or less from the time I started 'the work.'"
Her work began when she heard God tell her to open a mission in Calcutta. The book includes her Jan. 13, 1947 letter in which she wrote to the Archbishop of Calcutta to request permission to found her own order, the Missionaries of Charity.
Several years later, she composed a letter as an exercise from her spiritual adviser to express her devotion to Jesus and passionately wrote, "I want to satiate your thirst with every single drop of blood that you can find in me. Don't allow me to do you wrong in any way." Click here to read the letter (courtesy of Doubleday).
To millions her work still shines as the example of Christlike devotion. It brought her the Nobel Peace Prize and beatification by Pope John Paul. But once she began her work in India she never heard God's voice again. Nine years after she founded her mission in Calcutta she wrote, "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."
"Even the sisters around her had no idea of the length and the depth," Kolodiejchuk said.
As many Catholics learn how long she suffered this crisis of faith, they are even more awed by her deeds.
"Unlike the other saints, who might have been going through their day with a lot of consolation from their prayer, Mother Teresa was running on empty and doing all these wonderful works," said Father James Martin.
But while the faithful see her struggle as inspirational, some atheists are taking it as confirmation of their own rational doubts and proof that the faithless can display enormous benevolence.
"Of course nonbelievers all over the world display compassion," said Dan Barker of the Freedom From Religion Foundation. "She was forced to go through the motions and admitted her own hypocrisy."
Ten years after her death, her Missionaries of Charity claims to have over a million volunteers comforting the sick and orphaned in 40 countries. This book is certain to stir those who pray the Vatican will canonize the nun from the slums. If it does, Mother Teresa may just be the patron saint of skeptics.