Some people are surprised that Mother Teresa had such a long dark night of the soul. Others are calling her inner struggle a crisis of faith. Both groups are silly. God chastises every son (and daughter) that He loves. Every great saint, except for the ones who were martyrd quickly had a dark period.
St. Therese fought a huge battle as she lay dying of TB. St. John of the Cross suffered greatly. St. Teresa didn't have it easy either. Fr. Corapi once talked about the great spiritual graces he was granted when he first reverted back to the Faith. And then, when he was no longer a "baby" Christian, the special favors ceased. Fr. Donald Callaway talks about the incredible, special moments he had with Jesus and Mary when he first converted. And then the marvelous moments stopped and he had to ask himself was it Jesus he loved or was it the graces. He was able to say it was Jesus and he became a priest.
Mother Teresa went through an intense period of dryness and yet, she still carried on for the love of Jesus. She did not see Him anymore. She didn't get any visits from angels of His blessed Mother but Teresa carried on. She trusted and she loved. That's a saint.