Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Provocative, Critical, Deconstructive
Graham Greene would have made a good blogger—he knew how to push buttons and provoke. The Power and the Glory is obviously critical of religious persecution, but Greene seems to hit hardest when narrating the faults and failings of the Church. The words "pious" and "piety" are not meant as virtues:
He wondered why she was here—probably for having a holy picture in her house. She had the tiresome intense note of a pious woman. They were extraordinarily foolish over pictures. (2.3.79)
The point isn't that religious pictures are bad, but that they're not worth a night in prison. Catholicism uses pictures in worship, but worship can still occur just fine without them. Hence the priest's criticism of the pious woman in the jail cell with him.
Greene, of course, is critical of more than pious women who love religious artifacts. We get to know only two priests: one's a cowardly drunk and the other is a cowardly vow-breaker. Your run-of-the-mill Catholic is no better. An old man insists the priest hear everyone's confessions despite the fact that everyone—including the priest—is super tired. A woman who shares the priest's jail cell scolds him because he won't condemn a pair of prisoners in the packed cell who are having sex—she threatens to write his bishop. No wonder the novel provoked a negative response from the Vatican.
The novel's provocative tone might have helped it to achieve lasting success. Because it's just as critical of the religious characters as of the characters persecuting them, it has a mature moral complexity. This isn't a sanitized-for-children story about a saint; it's a gritty tale that blurs the line between saint and sinner better than Ariana Grande.