Defining oneself as "not that"

[F]rom 1517 on, and especially during the next century and a half, all changes in religion were shaped as much in reaction or opposition to [other Christian groups] as they were by any internal logic. Protestant self-definition relied on identifying what was wrong or false in Catholicism. And vice versa, Catholic self-definition strained to be the polar opposite of all things Protestant. Moreover, within Protestantism this same process played itself out in two directions, for it was just as important for each of the various Protestant churches to distinguish what was wrong or false within all the other opponents of the pope, as it was to prove that the Catholic Church was totally wrong. This constant process of defining what was true and right in opposition to what was false and wrong became inescapable, even for reforming groups that sought to live in isolation from the rest of the world. Exclusion is as important in religion as inclusion, and this is especially true of the Christian religion, which has always cared immensely about determining and clearly defining what makes the true different from the false, the genuinely faithful different from all that is deviant, the communion of saints different from all others.

-- Carlos M. N. Eire, Reformations : The Early Modern World, 1450-1650, New Haven, Conn. : Yale University Press (2016), pg. xi