"De Civitate Dei"
Saints having doubt about their own faith is far from unique. St. John of the Cross wrote extensively on what he called "the dark night of the soul." St. Faustina wrote in her diary of numerous periods of feeling abandoned by Christ. Mother Theresa's struggle seems to further the evidence that she had chosen the better love Augustine wrote about to the point where she lost herself completely. And she accomplished this loss in the midst of an environment where one would likely retreat into self as an escape from the horror, pain, and misery that defines the total poverty of Calcutta. How many actually see Christ in the faces of those who live their lives in the gutter? Mother Theresa embraced them. In a place where God seems to have abandoned His children, how many would actually aspire to bring His love and be the light in so much darkness? Mother Theresa gave all of her light to these poor souls. Small wonder that when she looked back into herself, the light of her own soul, the light of Christ alluded her. Perhaps it had all illuminated out of her very being. In such a case, doubt appears a completely reasonable response.
The challenge for many rests in the assumption that faith and reward go hand in hand in their earthly existence. If one believes in God, it stands to reason that God blesses that faithful one with the appropriate material comforts. Surely there must exist a reason for all of the efforts of faith, and human logic naturally points to the direction of a quid pro quo. Oh, sure, Heaven is a reward for faith; however, the afterlife remains nebulous and a far off distant place. What about the here and now? What about me?
"Strive to enter by the narrow door; for many, I tell you, will seek to enter and will not be able."
At the very least, Mother Theresa's doubt reveals her humanity. She cooperated with the grace of Christ in the fullest way a human being could. She was not supernatural or super-human. She was one, tiny woman who demonstrated with her life what could be achieved by simply saying yes to Our Lord without any strings attached.
Perhaps the pundits who attempt to disparage her life with their passive aggressive jabs at the authenticity of her sincerity are those who engage in Augustine's struggle of the two loves, but instead of fighting for God, they battle for self. That is a poverty that only the grace of Christ can overcome.