a mainstream religion that predominates throughout the western world. it purports to follow the teachings of jesus christ/jesus of nazareth, a sectarian leader from roman-occupied judea some 2000 years ago, regarded as a divine figure (either god or a relative of His) by christians. jesus taught many things, but since almost nobody can agree on what exactly they were, discussing these things is a bit difficult.
christianity comes in a number of flavors.
--catholicism: more or less original recipe christianity, supposedly founded by jesus' friend peter, who tried to bring his teachings to rome and in doing so was crucified upside down and became known as the first pope. catholics favor big cathedrals and elaborate vestments, and a complicated hierarchy of bishops, priests, deacons, monks and nuns, and thus they're the only denomination that gets to fight demons in the movies. practically any denomination that is not catholic is protestant.
--other episcopal denominations, who like the vestments and complicated hierarchy but don't want to follow the pope, resulting in a number of schisms. includes the egypt-based coptic church which broke away in 42 AD; the greek-based eastern orthodox church, which broke away in 1054; the german-based lutheran church that broke away in 1517; the england-based anglican church which broke away in 1534; and the episcopaleans who are basically anglican but refuse to acknowledge anything from england.
--then there's a bunch of denominations that don't like having elaborately-dressed bishops or cathedrals at all, so they settle mostly for button down shirts and office buildings: see presbyterians, baptists, and the red-headed stepchild of the family tree, the mormons
as might be inferred, it's a whole big thing.