Honore de Balzac 
Atheist’s Mass [EPUB ebook] 

The main character, Desplein is a successful surgeon and an atheist. His former assistant and friend is Doctor Horace Bianchon. One day Bianchon sees Desplein going into the Saint-Sulpice church, and follows him. He sees Desplein alone attending a mass. After Desplein departs, Bianchon questions the priest and finds that Desplein attends a mass at the church four times a year which he himself pays for. (Wikipedia)