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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."