The English word pagan is from the Latin paganus or someone who lives in a rural district, or pagus. In Latin, the word meant a villager or rustic, and was also used as an antonym for miles, or soldier. So when applied to a person, paganus means civilian.
The word makes its English appearance in the 14th century. The original English sense is the same as we use it today, meaning someone not belonging to society’s dominant religion, specifically a non-Christian. From Thomas Malory’s Morte Arthure, probably written sometime before 1400:
I sall...euer pursue the payganys þat my pople distroyede.
(I shall...ever pursue the pagans that my people destroyed.)
How it made the transition from the Latin for rustic or civilian to the English meaning is uncertain. Generally, one three explanations is proferred.
The first is that the English sense is a development from the rustic sense. As Christianity spread in the cities of the Roman empire, in the countryside the worship of the Roman gods continued for much longer. So those from the countryside were less likely to be Christians.
The second is that it is a development of the civilian sense. Christians called themselves milites, or soldiers of Christ. Pagans were the opposite.
Finally, again from the rustic sense, it came from the idea that those in the countryside were not part of urban society. They were a people apart. The metaphor was applied to religion as well, a people apart from the community of Christians were pagans.
(Source: Oxford English Dictionary, 3rd Edition)
Copyright 1997-2016, by David Wilton