By Mahita Gajanan
September 2, 2016

Mother Teresa will become a saint on Sunday, less than a year after Pope Francis certified her second miracle, a requirement for sainthood.

During her lifetime, she was often referred to as a “living saint,” winning the 1979 Nobel Peace Prize for her work with the Missionaries of Charity helping poor and sick people. However, Mother Teresa’s seemingly unbreakable bond with God was much more complicated than she let on in public. For nearly 50 years, she felt God had abandoned her.

Letters made public years after her death in 1997 revealed that Mother Teresa spent nearly half a century without feeling God’s presence, “neither in her heart or in the eucharist,” as TIME reported in 2007:

Mother Teresa was aware of the contradiction between her public persona and private feelings. She called her smile “a mask” or “a cloak that covers everything.” Her letters reveal she wrestled with the existence of God. In an undated prayer to Jesus at the suggestion of a confessor, she said she had no faith.

Such a lengthy crisis of faith becomes all the more significant as Mother Teresa is declared a saint. “I’ve never read a saint’s life where the saint has such an intense spiritual darkness,” Rev. James Martin, author of My Life with the Saints, told TIME in 2007. “No one knew she was that tormented.”

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The revealing letters were published in a book entitled Mother Teresa: Come Be My Light, compiled and edited by Rev. Brian Kolodiejchuk, Mother Teresa’s postulator. In an interview with the New York Times this August, Kolodiejchuk called Mother Teresa’s perseverance “heroic” in the face of the the “darkness” she felt.

“She was suffering that loneliness, that sense of being unloved, unwanted in her relationship with Jesus,” he said. “but in solidarity with and identified with others who were in some way living that sense of loneliness and being unloved.”

Read TIME’s full 2007 cover story about Mother Teresa’s letters, here in the TIME Vault: The Secret Life of Mother Teresa

Write to Mahita Gajanan at mahita.gajanan@time.com.

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