How we cite our quotes:
He was found dead, his body drooping over the bedside […]. Against his name the index card recorded: doubtful case.
Doubtful case! Exactly. While the index card technically refers to the priest’s doubtful diagnosis regarding whether or not he had the plague, it also refers to the doubt we (and the rest of the church, and probably Paneloux himself) have as to the nature of his faith.
"It comes to this," Tarrou said almost casually; "what interests me is learning how to become a saint."
"But you don’t believe in God!"
"Exactly! Can one be a saint without God?—that’s the problem, in fact the only problem, I’m up against today." (4.6.38-40)
Tarrou wonders if he can define "saint" in a secular manner. What appeals to him about the word is most likely moral, not religious: the selflessness, the virtue.