While no one knew it at the time, 1951 was a pivotal year for Mother Teresa of Calcutta, the start of a private battle for the tiny nun millions hailed as a living saint.

"When we talk about Mother Teresa we celebrate her victories and all the good works she accomplished in her life. But what did this victor have to overcome? That's an important question," said journalist Kenneth Woodward, author of "Making Saints: How The Catholic Church Determines Who Becomes A Saint, Who Doesn't, and Why."

"We often miss this spiritual warfare component in the lives of the saints, that whole element of struggle and grace. … With Mother Teresa, this just has to be there or her story is not complete."

It was in 1928 that 18-year-old Agnes Gonxha Bojaxhiu left her family in Macedonia to join the Sisters of Our Lady of Loreto, first working as a teacher in Calcutta.

Then, on Sept. 10, 1946, Sister Mary Teresa experienced a vision of Jesus calling her to move into the slums while serving the poorest of the poor. After this "call within a call" she created the Missionaries of Charity, beginning the work that produced waves of support for the Vatican to proclaim her a saint -- which will occur in rites on Sept. 4, the eve of the anniversary of her death on Sept. 5, 1997.

But another story was unfolding that remained a secret for decades.

It was in 1951 that Mother Teresa prayed that she be allowed to share the pain and loneliness that Jesus suffered on the cross. Her private letters made it stunningly clear that this prayer was granted. Her visions stopped, replaced by silence.