"Honestly, Hinkley, of all the Christians I ever met you take the rottenest advantages. You can lick anybody in the class, and when I think of how you're going to bully the poor heathen when you get to be a missionary […] I could bawl!" (2.5.9)
Martin knows that his classmate Ira Hinkley is going to have a ton of success when it comes to converting people to Christianity. The guy is so huge and full of energy that he'll be perseverant in his missionary work.
Gottlieb, the placidly virulent hater of religious rites, had a religious-seeming custom. (13.2.1)
Even though Gottlieb hates organized religion, he has personal rituals that can often seem religious. One of them is kneeling at the side of his bed and letting his mind wander. But he doesn't think about God or miracles. He just marvels at the power of his own scientific mind.
Often he knelt by his bed and let his mind run free. It was very much like prayer, though certainly there was no formal invocation, no consciousness of a Supreme Being—other than Max Gottlieb." (13.2.2)
There's really not much to Max Gottlieb's "praying," other than an admiration for himself and his own pursuit of scientific truth. At the same time, this book still refers to his devotion to science as a form of religion.
"And a fellow from down there was telling me Arrowsmith is great on books and study, but he's a freethinker—never goes to church. (17.1.35)
Nowadays, it might be a compliment to call someone a freethinker. But back in Sinclair Lewis' day, this wasn't necessarily the case. Being a freethinker was seen as a negative quality, and a sign that you were living an improper life.
One of its smallest but oldest industries is Mugford Christian College […] It has never been disgraced by squabbles over teaching evolutionary biology—it never has thought of teaching biology at all. (19.1.7)
Martin knows what kind of town he's moving to when he realizes that their main college would never allow a professor to teach evolution in the classroom. In fact, they don't bother teaching biology at all. According to them, you don't need to know how the human body works. God made it perfectly from scratch, and that's all you need to worry about.
"I make many mistakes. But one thing I keep always pure: the religion of a scientist." (26.1.25)
Max Gottlieb knows he's not perfect. But he's confident that he has always remained true to his true religion, which is science. He's not even interested in using science to cure diseases. He only cares about truth for its own sake, and will study anything that interests him, regardless of whether other people will ever benefit from it.
"God give me strength not to trust to God!" (26.2.3)
In his times of true despair, Martin wants more than anything not to turn to religion. That's why he asks God to give him the strength not to believe in God. But of course, you can see a bit of a contradiction here, since he's asking for help from someone he's not supposed to believe in.
If it was the intervention of the Lord […] had not the Lord surely sent him? (35.7.2)
The people of St. Hubert are convinced that Martin Arrowsmith was sent by God to save them. How else could they explain the fact that this guy showed up with a cure for the plague out of nowhere? According to this passage, people with religious minds will always interpret things as being part of a bigger Grand Plan.
[But] these acerbities made up the haircloth robe wherewith [Terry] defended a devotion to such holy work as no cowled monk ever knew. (39.1.36)
Once again, the book compares the pursuit of scientific truth to religious devotion. Terry Wickett is a disciple of science, just like Martin Arrowsmith and Max Gottlieb. But Terry seems to go even farther at times. He has absolutely no confusion about his devotion to science, while Martin can sometimes waffle between science and his marriage.
"The righteous, even the Children of Light, they shall be rewarded with a great reward and their feet shall walk in gladness, saith the Lord of Hosts; but the mockers, the Sons of Belial, they shall be slain betimes and cast down into darkness and failure, and in the busy marts shall they be forgot." (40.4.6)
In the final lines of this book, we look in on Bertie Tozer, who's giving a little speech about people who go to heaven and those who go to hell. This last look at Bertie reminds us that even as Martin Arrowsmith works away in a laboratory, there are tons of people all around the world who consider him blasphemous and believe he might go to hell.