I am never sure what this story about. Supposedly it was about an affair, but the central actress of the affair died too soon to my taste, and I am left with the anguish and sadness of the lover for too long. A story about an affair is supposed to expose a conflict between the lover and the husband, at the end it seems the conflict was between the lover and God. It was enjoyable nevertheless. You do not need to comprehend a story to like it.
Storytellers who hold on to the doctrine of "show, don't tell", will be perplexed how Graham Greene managed to break the rule and still write quite well. Perhaps because he was British, and the rule, IIRC, was coined by Americans. The conflicts in the story are largely internal. Every characters behave quite politely to each other without much drama. Only at key scenes some characters let her or his emotion blow up. Sometimes silence is much effective in conveying the pain felt by the protagonist to the readers.