How did infallibility become part of the papacy?
It’s important to understand that papal infallibility does not cover everything the pope does and says. That is a very common bumper-sticker sort of misconception, but infallibility is only for those rare occasions when he is speaking ex cathedra (from the chair). The ex cathedra statement is on matters of core Church doctrine, e.g. the Virgin Birth or transubstantiation, that are meant to be a solid statement of dogma. It is held to be a matter on which God has directly inspired the Pope to speak. It’s very rare for a Pope to invoke that, as you may imagine. The concept was made part of the Pope’s authority by the first Vatican Council of 1870. It hasn’t been used since 1950, when Pope Pius XII declared that Mary had been bodily assumed into Heaven, making it part of official Catholic required belief.
