I think the criticisms came from two sources. The first was that she made statements that made it look like she didn't believe that it was important for a person to trust in Jesus for their salvation as long as they lived a good life showing love to others.
"There is only one God and He is God to all; therefore it is important that everyone is seen as equal before God. I’ve always said we should help a Hindu become a better Hindu, a Muslim become a better Muslim, a Catholic become a better Catholic. We believe our work should be our example to people. We have among us 475 souls - 30 families are Catholics and the rest are all Hindus, Muslims, Sikhs—all different religions. But they all come to our prayers."
The other source of criticism came when it was revealed that for many years she was just going through the motions of being saintly but had no real faith in her heart to comfort her.
The letters, written by Mother Teresa in the 1950s and 1960s to her church spiritual guides, also reveal the troubling and, at times, painful conflicts she sometimes had with her faith.
"I am told God lives in me -- and yet the reality of darkness and coldness and emptiness is so great that nothing touches my soul," she wrote in one of the letters.
Some of the letters depict a spiritually bereft Mother Teresa, struggling to maintain her belief.
"Where I try to raise my thoughts to heaven, there is such convicting emptiness that those very thoughts return like sharp knives and hurt my very soul. Love -- the word -- it brings nothing," wrote the woman known the world over as the "Messiah of Love."
In another letter, Mother Teresa wrote, "In my soul, I can't tell you how dark it is, how painful, how terrible -- I feel like refusing God."