Mother Teresa et. al.

I've been reading lots of online posts from atheists condemning religion as the cause of almost every atrocity in history and arguing that systems of ethics formulated without reference to any idea of the divine are by definition more humane.  I do not question that there are ethical atheists and that there may even be ethical systems based in an atheistic understanding of the universe that are positive.  I also have no illusions about the history of religion and know that many atrocities have been and continue to be committed in the name of just about any religion you can think of. 

So, here's my question... if atheism can produce the same or higher quality of person as a religious system why is it that we see no atheist Mother Teresa's, Gandhi's, or the like?  Granted the list of such folk coming from religious backgrounds is not a huge one, but it does exist and includes many less famous folk who have given themselves to make the world a better place.  I'm not aware of any atheists who have made similar ethical impact in the world.  On the other hand, we can find atheist leaders who have committed the same kinds of horrific violence that has been attributed to religious folk.

What do you think?