heathen
A person seen as not following a main religion.
Heathen originally meant someone who didn't follow Christianity, especially in ancient or medieval times.
Vikings and people from ancient Greece and Rome were all called heathens by medieval Christian writers because they worshipped different gods like Odin, Zeus, or Jupiter. In historical books, you might read about missionaries traveling to convert “heathen peoples” to Christianity.
Today, the word is rarely used seriously in this religious sense. When people do use it, it's often playfully or jokingly, like calling your friend a “heathen” for putting ketchup on fancy steak, or for not knowing who won the World Series. In this casual use, heathen just means someone who doesn't follow certain customs or traditions that others take seriously.
The word carried judgment in the past, suggesting that people who believed differently were somehow less civilized. Modern historians and anthropologists recognize that ancient cultures once labeled “heathen,” like the Vikings or Maya, built sophisticated societies with their own wisdom, art, and achievements. Some people today who practice ancient Norse or Celtic religions actually call themselves heathens with pride, reclaiming the word for their spiritual traditions.