While gathering biographical material for his case to the Vatican that Mother Theresa deserves sainthood, a priest finds that the Nobel Prize winner was, if not agnostic, then certainly riddled with doubt about the existence of God. 

Wow.  And you think you know a saint.  Er, potential saint.

Her doubts appear in letters and private correspondence to friends:

Shortly after beginning work in Calcutta’s slums, the spirit left Mother Teresa.

"Where is my faith?" she wrote. "Even deep down… there is nothing but emptiness and darkness… If there be God — please forgive me."

Eight years later, she was still looking to reclaim her lost faith.

"Such deep longing for God… Repulsed, empty, no faith, no love, no zeal," she said.

As her fame increased, her faith refused to return. Her smile, she said, was a mask.

"What do I labor for?" she asked in one letter. "If there be no God, there can be no soul. If there be no soul then, Jesus, You also are not true."

By the time of her death, she was still plagued with doubts, and had even stopped praying.

Seeing the things she had seen, it hardly surprises me that anyone would question the existence of a loving God.

As Time notes, Mother Theresa "considered the perceived absence of God in her life as her most shameful secret", so much so that she asked that her letters be burned after her death.  (Obviously, this didn’t happen).

I don’t know that there’s any reason for shame.  She started a mission for, in her own words, "the hungry, the naked, the homeless, the crippled, the blind, the lepers, all those people who feel unwanted, unloved, uncared for throughout society, people that have become a burden to the society and are shunned by everyone."  By the time of her death, she was operating 517 missions in over 100 countries, including 20 in the United States.  Nothing to sneeze at.