Honoré de Balzac 
An Episode under the Terror [PDF ebook] 

Un épisode sous la Terreur (English "An Episode during the Terror") is a short story by Honoré de Balzac, published in 1830.
An elderly woman buys a package from a bakery on 22 January 1793, and suspects she is being followed. She asks for protection from the baker, but when he sees the stranger following her he refuses. She returns home alone, and it is revealed that she is an ex-Carmelite nun in hiding with another nun and a priest. All are elderly, and the box she bought contains communion wafers. The stranger comes up to their room, and asks the priest to say a mass for the recently executed King Louis XVI. He returns later to attend the mass. He also says that he will come back in a year for another mass for the king, and promises that they will be safe from any danger. He leaves a blood stained handkerchief with royal insignia as a present.
The next year the stranger returns for the mass. Later, the priest visits a shop owned by some Royalist friends of his, and notices a tumbril passing by with the executioner and his victims. The priest faints when he recognises that the executioner is the stranger who has been helping them, and realises that the handerkerchief was the king's.
The story takes place during the Reign of Terror, from 1793 to 1794. It starts the day after the execution of King Louis XVI. The story is fictional, but it features an historical figure, Charles-Henri Sanson, who was the High Executioner, and conducted the execution of Louis XVI. However the story does not give his name.
The Reign of Terror was a period during which there was a process of Dechristianisation occurring in France. The nuns and priest in the story are in hiding because of this.