It was not invented by Christians. It is a regular ol' Latin word and therefore goes all the way back to Proto-Indo-European (see below). In pre-Christian Latin it is first and foremost just an adjective, inferus/a/um, that indicates "below, underneath, lower." It is completely generic: it can refer to a jelly donut, or any other mundane thing: in Varro, aqua infera, "lower rainwater (going into the drain)"; the mare infernum is the "Lower" ie Tuscan Sea, while the mare superum is the "Upper" ie Adriatic Sea. But yes, it also frequently denotes the Underworld, the lower world, and those who dwell there. Jupiter and his Olympian crew are the deī superī, the gods up above, while the rulers of the Underworld are the deī inferī, the gods below (so in Plautus, Ovid, Vergil, etc). It can also act as a substantive adjective for the shades of the dead.

Like any regular Latin adjective, it has a comparative degree, inferior, inferiōris "lower, deeper" and a superlative degree, slightly irregular, and typically shortened to īmus/a/um "the lowest, deepest." These can also refer to time as well as space, with a "lower" time meaning later. Also in Latin is the derivative adjective infernus/a/um, meaning about the same thing "lower down," from which we get English words like "infernal" with all their scary connotations.

Interestingly, the word does not have many obvious cousins in other Indo-European languages. Nothing in Greek that is obviously related. Sanskrit has adhara, Avestin adara, "lower"; we have Gallic anderon, genitive plural "of the gods of the underworld" (uncertain); Gothic undar and Old High German untari "under". These are all from a presumed Proto-Indo-European root in ndhara-.