Doubt does not disqualify one from the community of believers; it simply serves to certify a person's membership in the human race. All humans, including all believers, have doubts.
Christians do well from time to time to recall the man who said to Jesus (Mark 24:9), "Lord, I believe; help thou my unbelief." This might be called the most Christian of all prayers. Jesus had just assured this man that his son, who was ill, would be alright if he, the father, just had faith. I have faith, I do believe, he replied, but I'm carrying with my faith a good measure of unbelief. I need help. As do we all.
Mother Teresa's experience was not what you would call classic atheism. She didn't deny the existence of God. It seems to me that her question, shared by many of us at different times, was not: does God exist? She struggled with a different question: does God exist right here with me at this time? It is an experience of darkness. It can be unimaginably frightening; it can lead to despair.
Doubt is an ongoing dimension of religious faith. Grace, another word for God's love, is stronger than the deepest doubt.