Note 173
From Chapter 21 of the Decline & Fall

As I have freely anticipated the use of pagans and paganism, I shall now trace the singular revolutions of those celebrated words.

1. in the Doric dialect, so familiar to the Italians, signifies a fountain; and the rural neighbourhood which frequented the same fountain derived the common appellation of pagus and pagans (Festus sub voce, and Servius ad Virgil. Georgic. ii. 382).
2. By an easy extension of the word, pagan and rural became almost synonymous (Plin. Hist. Natur. xxviii. 5); and the meaner rustics acquired that name, which has been corrupted into peasants in the modern languages of Europe.
3. The amazing increase of the military order introduced the necessity of a correlative term (Hume's Essays, vol. i. p. 555); and all the people who were not enlisted in the service of the prince were branded with the contemptuous epithet of pagans (Tacitus "Histories" iii. 24, 43, 77. Juvenal. Satir. 16 [v. 32]. Tertullian de Pallio, c. 4).
4. The Christians were the soldiers of Christ; their adversaries who refused his sacrament, or military oath of baptism, might deserve the metaphorical name of pagans; and this popular reproach was introduced as early as the reign of Valentinian (A.D. 365) into Imperial laws (Cod. Theodos. 1.xvi tit. ii. leg. 18) and theological writings.
5. Christianity gradually filled the cities of the empire: the old religion, in the time of Prudentius (advers. Symmachum, l. i. [v. 575 sqq.] ad fin.) and Orosius (in Praefat. Hist.), retired and languished in obscure villages; and the word pagans, with its new signification, reverted to its primitive origin.
6. Since the worship of Jupiter and his family has expired, the vacant title of Pagans has been successively applied to all the idolaters and polytheists of the old and new world.
7. The Latin Christians bestowed it, without scruple, on their mortal enemies the Mahometans; and the purest unitarians were branded with the unjust reproach of idolatry and paganism. See Gerard Vossius, Etymologicon Linguae Latinae, in his works, tom. i. p. 420; Godefroy's Commentary on the Theodosian Code, tom. vi. p. 250; and Ducange, mediæ, et infimæ Latinitat. Glossar.