France in 1337 was mainly Roman Catholic. Catholicism was the dominant religion in the Kingdom of France during that period, and it was closely tied to the monarchy and wider medieval European Christianity.

Quick context

  • The Hundred Years’ War began in 1337.
  • France at that time was a largely Catholic kingdom.
  • Other religions existed, but Roman Catholicism was the main one.

Simple answer

If you want the shortest version: Roman Catholicism.

TL;DR: In 1337, France’s main religion was Roman Catholicism.