The word pagan was actually a piece of Roman military slang
Before it described ancient gods, 'pagan' was a derogatory label for civilians and peasants. It was an exonym—a name imposed by outsiders—created by early Christians to distinguish their 'soldiers' from those left behind in the rural provinces.
The term 'pagan' originates from the Latin *paganus*, a word that originally lacked any religious significance. Depending on the context, it could refer to a country dweller, a rustic, or even a 'non-combatant' in Roman military jargon. As early Christians began to adopt the persona of *milites Christi*, or 'soldiers of Christ,' they likely repurposed this secular term to describe those who remained 'civilians'—those not part of their religious movement.
Historians debate exactly how this linguistic shift occurred. One theory suggests it reflected the geography of conversion: as Christianity urbanized, the 'peasantry' in remote, rural districts (*pagus*) maintained traditional polytheistic practices longer. Another possibility is that it was a purely military distinction used by soldiers stationed in the provinces. By the mid-4th century, the religious connotation was firmly established, used to categorize anyone who did not worship the God of Abraham.
In the Greek-speaking East, the terminology differed. Christians used terms like *ethnikoi* (foreigners) or *Hellēnes* (Greeks) to denote non-believers. This created a fascinating linguistic paradox where 'Hellenism' became a religious label rather than a purely cultural one. While the term was historically used as a pejorative to imply 'false gods' or 'unlearned' ways, the 20th century saw a significant shift. Modern practitioners have reappropriated the label, transforming a tool of religious exclusion into a self-chosen identity for various polytheistic and nature-based traditions.
Source: Paganism
