I have just started reading The Ball and the Cross by G.K. Chesterton (having only read about 10 pages so far)
As Martin Garnder describes the plot of the novel in his Introduction to the Dover edition:
Evan MacIan is a tall, dark-haired, blue-eyed Scottish Highlander and a devout but naive Roman Catholic. He is so politically conservative that he is a Jacobite who longs for a restoration of the Stuart monarchy. James Turnbull is a short, red-haired, gray-eyed Scottish Lowlander and a devout but naive atheist. Politically he is a romantic socialist...Both men are fanatical in their opinions.
The two meet when MacIan smashes the window of the street office where Turnbull publishes an atheist journal. This act of rage occurs when MacIan sees posted on the shop's window a sheet that blasphemes the Virgin Mary, presumably implying she was an adultereress who gave birth to an illegitimate Jesus. When MacIan challenges Turnbull to a duel to the death, Turnbull is overjoyed. For twenty years no one had paid the slightest attention to his Bible bashing. Now at last someone is taking him seriously! Most of the rest of the story is a series of comic events in which the two enemies wander about seeking a spot for their duel, only to be forever prevented from fighting by the police and other civil authorities.
Two aymmetrical romances enter the narrative. MacIan, the believer, falls in love with Beatrice, an Englishwoman who is an agnostic. Turnbull, the skeptic, is smitted with Madeleine, a Frenchwoman who is a practicing Catholic. At the end, each man comes to admire the other for the courage of his convictions, and the become good friends even as they continue their efforts to fight...
Gardner (who describes himself as a "philosophical but non-Christian theist") states, one may
...enjoy reading [the novel] for its colorful style, with its constant alliteration, amusing puns and clever paradoxes; for its purple passages about sunsets, dawns and silver moonlight; and for the humor and melodrama of its crazy plot