Like most illustrations in children's books, the image of Mother Teresa is quite simple, showing her kneeling in prayer beside her bed in a dark room, facing a bare cross and a single candle.

The tiny nun's eyes are open and her expression is hard to read. The text on the opposite page is candid.

"Mother Teresa experienced a great sorrow. Ever since she had moved to the slums, she no longer felt the presence of Jesus as she had before. She felt as though abandoned, rejected by him," according to "Mother Teresa: The Smile of Calcutta," a storybook for young children. "In her heart, she felt darkness and emptiness. She experienced the suffering of the poor who did not feel loved. She shared in the loneliness Christ suffered on the Cross."

Only the priests who worked with her knew about this "dark night of the soul," an experience seen in the lives of some other saints.

Working with text by Charlotte Grossetete, originally written in French, Ignatius Press editor Vivian Dudro said she "spent lots of time working on how to phrase that part. … You picture a young child reading about this pain in a saint's life or having this story read to them. How do you explain something like this in a few simple words?"

This dark night is clearly a crucial part of the life of the Albanian nun who was canonized this past weekend as St. Teresa of Kolkata. The formal petition to Pope Francis concluded: "Despite a painful experience of inner darkness, Mother Teresa travelled everywhere, concerned … to spread the love of Jesus throughout the world. She thus became an icon of God's tender and merciful love for all, especially for those who are unloved, unwanted and uncared for."

St. Teresa's sense of spiritual loss was the mirror image of the intense spiritual visions that, in 1946, inspired her to plunge deep into the slums of Calcutta (now called Kolkata) to serve the poorest of the poor.