Quote 1
"Like the ore that must be melted all to liquid to find the pure metal, so must we be rendered in the fiery furnace of this disease." (2.6.17)
Once again, we see Mompellion argue that suffering is a good thing. It's like a daily vitamin. While he might not be entirely incorrect, this passage becomes increasingly ironic as the novel goes on. Check out "Quote #10" in this section for more insight into this issue.
Quote 2
"If God saw fit to send this scourge, I believe it would be His will that one faces it where one was, with courage and thus contain its evil." (2.3.48)
Not everyone thinks that suffering is a bad thing. Mompellion, true to his Puritan belief system, sees suffering as God's way of testing his people. If that's so, then consider the plague God's equivalent to the MCAT. You've got to study hard for that sucker.
Quote 3
"I thought I spoke for God. My whole life, all I have done, all I have said, all I have felt, has been based upon a lie. Untrue in everything." (3.15.89)
Our brains get blown when Mompellion drops this bomb near the end of the novel, basically saying that he no longer believes in God after Elinor's death. He was able to rationalize suffering when it was somewhat separate from him, but now that it's hit close to home, he doesn't know how to deal.