Paganism (from classical Latinpāgānus "rural, rustic," later "civilian") is a term first used in the fourth century by early Christians for people in the Roman Empire who practiced polytheism. This was either because they were increasingly rural and provincial relative to the Christian population, or because they were not milites Christi (soldiers of Christ). Alternate terms in Christian texts for the same group were hellene, gentile, and heathen. Ritual sacrifice was an integral part of ancient Graeco-Roman religion and was regarded as an indication of whether a person was pagan or Christian.
The origin of the application of the term pagan to polytheism is debated. In the 19th century, paganism was adopted as a self-descriptor by members of various artistic groups inspired by the ancient world. In the 20th century, it came to be applied as a self-descriptor by practitioners of Modern Paganism, Neopagan movements and Polytheistic reconstructionists. Modern pagan traditions often incorporate beliefs or practices, such as nature worship, that are different from those in the largest world religions.
It is crucial to stress right from the start that until the 20th century, people did not call themselves pagans to describe the religion they practised. The notion of paganism, as it is generally understood today, was created by the early Christian Church. It was a label that Christians applied to others, one of the antitheses that were central to the process of Christian self-definition. As such, throughout history it was generally used in a derogatory sense.
The adoption of paganus by the Latin Christians as an all-embracing, pejorative term for polytheists represents an unforeseen and singularly long-lasting victory, within a religious group, of a word of Latin slang originally devoid of religious meaning. The evolution occurred only in the Latin west, and in connection with the Latin church. Elsewhere, Hellene or gentile (ethnikos) remained the word for pagan; and paganos continued as a purely secular term, with overtones of the inferior and the commonplace.
Medieval writers often assumed that paganus as a religious term was a result of the conversion patterns during the Christianization of Europe, where people in towns and cities were converted more readily than those in remote regions, where old ways lingered. However, this idea has multiple problems. First, the word's usage as a reference to non-Christians pre-dates that period in history. Second, paganism within the Roman Empire centred on cities. The concept of an urban Christianity as opposed to a rural paganism would not have occurred to Romans during Early Christianity. Third, unlike words such as rusticitas, paganus had not yet fully acquired the meanings (of uncultured backwardness) used to explain why it would have been applied to pagans.
Paganus more likely acquired its meaning in Christian nomenclature via Roman military jargon (see above). Early Christians adopted military motifs and saw themselves as Milites Christi (soldiers of Christ). A good example of Christians still using paganus in a military context rather than religious is in Tertullian's De Corona Militis XI.V, where the Christian is referred to as paganus (civilian): 
Apud hunc [Christum] tam miles est paganus fidelis quam paganus est miles fidelis.
With Him [Christ] the faithful citizen is a soldier, just as the faithful soldier is a citizen.
Paganus acquired its religious connotations by the mid-4th century. As early as the 5th century, paganos was metaphorically used to denote persons outside the bounds of the Christian community. Following the sack of Rome by the Visigoths just over fifteen years after the Christian persecution of paganism under Theodosius I, murmurs began to spread that the old gods had taken greater care of the city than the Christian God. In response, Augustine of Hippo wrote De Civitate Dei Contra Paganos ('The City of God against the Pagans'). In it, he contrasted the fallen "city of Man" to the "city of God" of which all Christians were ultimately citizens. Hence, the foreign invaders were "not of the city" or "rural".
The term pagan is not attested in the English language until the 17th century. In addition to infidel and heretic, it was used as one of several pejorative Christian counterparts to gentile (גוי / נכרי) as used in Judaism, and to kafir (كافر, 'unbeliever') and mushrik (مشرك, 'idolater') as in Islam.
This was influenced by Christianity's early members, who were Jewish. The Jews of the time distinguished themselves from foreigners according to religion rather than ethno-cultural standards, and early Jewish Christians would have done the same. Because Hellenic culture was the dominant pagan culture in the Roman east, they called pagans Hellenes. Christianity inherited Jewish terminology for non-Jews and adapted it in order to refer to non-Christians with whom they were in contact. This usage is recorded in the New Testament. In the Pauline epistles, Hellene is almost always juxtaposed with Hebrew regardless of actual ethnicities.
The usage of Hellene as a religious term was initially part of an exclusively Christian nomenclature, but some Pagans began to defiantly call themselves Hellenes. Other pagans even preferred the narrow meaning of the word from a broad cultural sphere to a more specific religious grouping. However, there were many Christians and pagans alike who strongly objected to the evolution of the terminology. The influential Archbishop of ConstantinopleGregory of Nazianzus, for example, took offence at imperial efforts to suppress Hellenic culture (especially concerning spoken and written Greek) and he openly criticized the emperor.
The growing religious stigmatization of Hellenism had a chilling effect on Hellenic culture by the late 4th century.
By late antiquity, however, it was possible to speak Greek as a primary language while not conceiving of oneself as a Hellene. The long-established use of Greek both in and around the Eastern Roman Empire as a lingua franca ironically allowed it to instead become central in enabling the spread of Christianity—as indicated for example by the use of Greek for the Epistles of Paul. In the first half of the 5th century, Greek was the standard language in which bishops communicated, and the Acta Conciliorum ("Acts of the Church Councils") were recorded originally in Greek and then translated into other languages.
The term has recently been revived in the forms Heathenry and Heathenism (often but not always capitalized), as alternative names for the Germanic neopagan movement, adherents of which may self-identify as Heathens.