Origin of PAGAN:
First Known Use: 14th century
There is no definition of "pagan" that applies to Christianity.
Your comrade, donsqueaker, disagrees with you.
The term pagan is from the Latin paganus, an adjective originally meaning “rural”, “rustic” or “of the country.” As a noun, paganus was used to mean “country dweller, villager, and contrasted with the religion in favor in the imperial court.
In that sense, Christianity is now a pagan religion. Christianity is, to my way of thinking, not in favor in our government. It is in favor in ‘flyover country’ which would be rural, rustic places, or in small villages.
Some (Muslims and Jews, and perhaps the occasional Roman Pagan) see the doctrine of the Trinity as approaching polytheism.
The Buddha said “The things of G-d are unknown, and unknowable, so why argue?”