apparently faith not a prerequisite for virtue

Turns out that Mother Teresa was plagued by doubts of God's existence. The evidence is found in a number of letters she wrote which have been collected by Rev. Kolodiejchuk, the priest who's making the case to the Vatican for Mother Teresa's proposed sainthood. Curiously, he said her obvious spiritual torment actually helps her case.

"Now we have this new understanding, this new window into her interior life, and for me this seems to be the most heroic," said Rev. Kolodiejchuk.

This confuses me. First off, how does her doubt make her heroic as a saint? I'll agree that her actions, her self-sacrifice were heroic (although some would disagree), but can she really be considered a saint? After all, her letters betray her faith. It will be interesting to see how the Vatican handles this one.