I won't be able to reproduce the layers of religious themes at work in the book, but perhaps a quote from the closing few pages captures how Greene sees sainthood tottering on the brink of demon possession (whereas Ida's world of moralistic certainty is actually never near to God). In the words of an anonymous priest:
He said gently, 'Corruptio optimi est pessima.'
'I mean--a Catholic is more capable of evil than anyone. I think perhaps--because we believe in Him--we are more in touch with the devil than other people.'