I have started reading the collection of Mother Teresa's letters. As I noted here and here, the press has been reporting that Mother Teresa was some sort of atheist, without realizing that her cry was that of an anguished believer.
The interesting thing so far is that she felt her call so strongly. In letters written to her spiritual advisor and to her Archbishop asking for permission to start the Missionaries of Charity, she described it so vividly. Christ asked her to minister to the poorest of the poor and she felt that she could do nothing but answer that call. Her superiors of course were not quite so convinced. Her spiritual advisor suggested that she take some time to reflect, her Archbishop suggested that they delay any decision, while her superiors transferred her to a remote school.
Yet she continued with pursuing her call. She wrote constantly to her Archbishop, requesting a decision (one that he tried to delay or pass to otters). In these letters, you can almost hear the pleading voice of the humble nun. Considering how vivid and strong the call was, you can understand why her later struggles were so devastating to her.
Perhaps that is the lesson of Teresa, that when we hear and answer the call, it is then up to us to make it happen. God points in the direction he wants us to go, but then it is up to us to get there.