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. “She understood very well when people would share their horror stories, their pain and suffering of being unloved, lonely. She would be able to share that empathy because she herself was experiencing it.”

Tens of thousands of people are expected for the canonization ceremony Sunday for the tiny, stooped nun who was fast-tracked for sainthood just a year after she died in 1997. St. John Paul II, who was Mother Teresa’s greatest champion, beatified her before a crowd of 300,000 in St. Peter’s Square in 2003.

Francis has made the canonization the high point of his Jubilee of Mercy, a yearlong emphasis on the church’s merciful side. Francis has an obvious interest in highlighting Mother Teresa’s mercy-filled service to outcasts on the periphery, given that her life’s work exemplifies the priorities of his own pontificate.

But Francis is also sending a more subtle message to the faithful through the canonization of the ethnic Albanian nun: That saints can be imperfect — they can suffer as Mother Teresa did and even feel unloved by God, said Ines Angeli Murzaku, a professor of church history at Seton Hall University in New Jersey and herself a native Albanian.

“That existential periphery which is suffering and being marginalized, he wants to bring that to the attention of the world,” she said in a telephone interview. Mother Teresa “is so real. She’s not remote. She’s not a perfect, perfect saint.”

That said, her blind faith in enduring the “darkness,” as she called it, and persevering through it seems almost superhuman to outsiders.

Take the Feb. 28, 1957 letter she wrote the then-archbishop of Kolkata, Jesuit Archbishop Ferdinand Perier.

“There is so much contradiction in my soul. Such deep longing for God, so deep that it is painful, a suffering continual, and yet not wanted by God, repulsed, empty, no faith, no love no zeal,” she wrote. “Souls hold no attraction. Heaven means nothing, to me it looks like an empty place. The thought of it means nothing to me and yet this torturing longing for God.”

“Pray for me please that I keep smiling at him in spite of everything.”

In another letter, she acknowledged that her smile was “a big cloak which covers a multitude of pains.”

Revelations that the smile was a mask to inner doubts about God’s presence fueled criticism of Mother Teresa — spearheaded most famously by the late Christopher Hitchens — that the Balkan nun was something of a fraud.

Kolodiejchuk, though, says she was no hypocrite. He said that the smile was a genuine and heroic attempt to hide her private sufferings, even from God, and prevent others from suffering more.

“You can be joyful even if you’re suffering because you are accepting, and you are working and acting with love that gives meaning to the suffering,” he said in the courtyard of one of the Missionaries of Charity houses on the periphery of Rome.

The revelations nevertheless shocked even Mother Teresa’s closest confidants and friends, the original sisters who joined her Missionaries of Charity after she was inspired to found the order in 1946. Kolodiejchuk said several sisters wept when he first read them her letters after he acquired them in 1998 from the archives of the Jesuits and archbishop in Kolkata.

Sister Prema, the current superior general of the Missionaries of Charity, recalled being in awe of the revelation and not being able even today to fully understand the depth of Mother Teresa’s pain.

“It took me some time, and it still takes me time, to reflect about it and to understand it more deeply,” she said in an interview. “I think a soul who has not experienced it (the darkness) will not be able to understand what it is about. This is some mystery of the spiritual life which souls who know about it can connect with and associate with, but souls who do not know, we stand before a mystery.”

Asked if she was in that latter group, the German nun paused and said quietly: “Yes.”

Kolodiejchuk, the postulator for the cause, says that in retrospect, Mother Teresa’s “darkness” was actually a critical part of her vocation, kept hidden from the world that only saw a firm but loving mother superior who was the first in the chapel each morning and often worked herself to exhaustion at night tending to society’s most unloved.

“We assumed at least she was enjoying this wonderful consoling union and love from Jesus,” he said. “But we discover, no it’s even the opposite. For me, this darkness is the single most heroic aspect of her life.”