VATICAN CITY — When Pope Francis canonizes Mother Teresa on Sunday, he’ll be honoring a nun who won admirers around the world and a Nobel Peace Prize for her joy-filled dedication to the “poorest of the poor.” He’ll also be recognizing holiness in a woman who felt so abandoned by God that she was unable to pray and was convinced, despite her ever-present smile, that she was experiencing the “tortures of hell.”

For nearly 50 years, Mother Teresa endured what the church calls a “dark night of the soul” — a period of spiritual doubt, despair and loneliness that many of the great mystics experienced, her namesake St. Therese of Lisieux included. In Mother Teresa’s case, the dark night lasted most of her adult life — an almost unheard of trial.

No one but Mother Teresa’s spiritual directors and bishop knew of her spiritual agony until her correspondence came to light during her beatification cause. The letters were then made available to the general public in a 2007 book, “Come Be My Light.”

For the Rev. Brian Kolodiejchuk, the Canadian priest who published the letters and spearheaded Mother Teresa’s saint-making campaign, the revelations were further confirmation of Mother Teresa’s heroic saintliness. He said that by canonizing her, Francis is recognizing that Mother Teresa not only shared the material poverty of the poor but the spiritual poverty of those who feel “unloved, unwanted, uncared for.”

“That was her experience in her relationship with Jesus,” Kolodiejchuk said in an interview.