Latin paganus: from pagus, country district, hence villager, also civilian as opposed to soldier. Early Christians, styling themselves milites Christi (soldiers of Christ), used paganus for everyone not enlisted, civilians of the faith. Rural districts kept old gods longest, so both meanings converged on the same word.
The retelling: Christian Rome sneered at countryfolk, and the sneer became doctrine. Heathen ran the identical play in the Germanic world, heath-dweller, the person out on the heath with the old shrines.
Pagus's family: pageant, and by a separate thread, propagate (originally to fix plants, from pangere). The countryside as the place where things stay rooted: old gods, old words, old plantings. The city's word for rural faith became the rural word for resistance.
Was Christianity an urban fashion before it was a faith?