An interesting piece describing that Mother Teresa apparently lost her faith in the end.
Charity Without Religious Belief: Mother Teresa | Cosmic Variance
Unfortunately, Mother Teresa seems to have despaired of her work because of that too: she should not have done so – helping other living beings is a means in itself; one does not need to do it for a higher entity.
This also shows a central flaw in the argumentation that ethics can only exist if there is a higher entity that “commands” what is good. How can something be good just because someone powerful says it is? That could never be a basis of ethics.
Something is good because we as living and feeling beings can discern between pleasure and pain; we know suffering and we know enjoyment. Our very physical being is the basis for our ethics, and helping others lead a better and more enjoyable life is good because of this very physical and real world – no higher spheres need be involved.