If that's a question then no, the Romans did not use the word pagan to refer to Christians.
The early usage (according to the OED) was
[ad. L. pāgān-us, orig. ‘villager, rustic; civilian, non-militant’, opposed to mīlēs ‘soldier, one of the army’, in Christian L. (Tertullian, Augustine) ‘heathen’ as opposed to Christian or Jewish. The Christians called themselves mīlitēs ‘enrolled soldiers’ of Christ, members of his militant church, and applied to non-Christians the term applied by soldiers to all who were ‘not enrolled in the army’. Cf. Tertullian De Corona Militis xi, ‘Apud hunc [Christum] tam miles est paganus fidelis quam paganus est miles infidelis’. See also Gibbon xxi.
Cf. payen.
The explanation of L. pāgānus in the sense ‘non-Christian, heathen’, as arising out of that of ‘villager, rustic’, (supposedly indicating the fact that the ancient idolatry lingered on in the rural villages and hamlets after Christianity had been generally accepted in the towns and cities of the Roman Empire: see Trench Study of Words 102, and cf. Orosius i Præf. ‘Ex locorum agrestium compitis et pagis pagani vocantur’) has been shown to be chronologically and historically untenable, for this use of the word goes back to Tertullian c 202, when paganism was still the public and dominant religion, and even appears, according to Lanciani, in an epitaph of the 2nd cent.]
1. One of a nation or community which does not hold the true religion, or does not worship the true God; a heathen. (†In earlier use practically = non-Christian, and so including Muslims and, sometimes, Jews.)
2. fig. or allusively. A person of heathenish character or habits, or one who holds a position analogous to that of a heathen in relation to Christian society.
etc.