[Ernesto]

Another fine point may be put upon this. Although in a general sense, 'pagus' refers to countryside and all of the above are true, there is a more specific Roman usage. The Roman city had not only physical walls and boundaries, but a spiritual boundary called the pomerium. This term is very ancient and derives from post (beyond) and murem (the wall). The wall in question is not a physical wall, but an imaginary sacred boundary denoted by stone cippi, or stakes marked as such. The pomerium began as a furrow plowed by Romulus on the Palatine Hill, and gradually expanded in fits and leaps as Rome itself expanded. However, not all real estate owned by the people and the state of Rome was within the pomerium or sacred city. For example, the Aventine Hill remained outside the pomerium for centuries, and contained the famous Temple of Diana, dedicated to the worship of deities not native to Rome but belonging to the Latin and Italian merchants who passed through the increasingly important commercial center that was Rome. Land belonging to the city of Rome, but not within the pomerium, was referred to as pagus. Thus, it undoubtedly was a term of disdain for some to use in referring to foreigners. Since the Christian god above all refused to make any accommodation with the deities native to Rome, the Christian god was utterly revulsive to patriotic and religious Romans. In fact, Christians were referred to as atheists for their lack of respect for the imperial and city cults, and their insistence on worshiping only one vague and powerless deity whose earthly incarnation perished as a common crucified criminal. It must have seemed a supreme irony, therefore, that when the Christians became the only authorized religion in the final days of the Western Empire, it was their turn to refer to their enemies as pagans--not only rustics, but those whose deities were not permitted within the (now no longer meaningful) pomerium.

http://en.wikipedia.org/wiki/Paganism