pagan
The word pagan comes from the Latin for "civilian." (Paganus also simply meant 'villager.')
In its religious sense, pagan refers to people who don't worship the God of monotheism.
So what's the connection between "civilian" and nonbeliever? It is speculated that pagan-as-civilian is a metaphorical formulation based on the idea of believing Christians as part of a spiritual army. When the earliest Church fathers spoke of Christian warriors they did not mean it literally.