It doesn't matter how much good she did. If you are guilty of associating partners with Allah, your place will be in Hell. And rightly so. This isn't injustice at all. People who are hurled into Hell deserve it. Now you might ask how do they deserve it when they did so many good deeds, helped so many poor people and gave so much in charity?
A verse in the Qur'an gives us the answer. It informs us that Allah is the creator of man and what he makes. This means all our deeds don't even belong to us. We would be deluding ourselves if we thought that we deserved to go to heaven because of our good deeds. Those good deeds are a gift given to us by Allah and we couldn't have done them if it hadn't been Allah's will or if Allah hadn't given us the ability or opportunities to do them.
So Mother Theresa might have done a lot of good deeds. But all those deeds have absolutely no weight since they were not dedicated to Allah alone. It doesn't even matter if you are a Muslim and you have done a lot of good deeds. If your deeds are not done sincerely for Allah, all those deeds will be rejected. What counts in the end is sincerity towards Allah.
That's with regard to good deeds, whether they are done by Muslims or non-Muslims. The quantity of deeds you do isn't at all important. It's the quality of deeds that matters.
But for a Muslim whose deeds are nullified or rejected because they lacked sincerity, he still has hope of being saved from the Fire, if he believes in Allah and his messenger (saws). Yes, his belief alone can save him from the Fire of Hell, provided of course that his belief is sincere.
For a non-Muslim, he has no hope whatsoever from being saved from the Fire. His good deeds belong to Allah, so he can't take credit for them. And second he doesn't believe in Allah and his messenger (saws), so there will be no god out there to save him from the eternity of Hell.
Lesson: Believe in Allah and his Messenger to obtain salvation from Hell.
La ilaha il Allah, Muhammaddar Rasul Allah!