infallibility, in Christian thought, exemption from the possibility of error, bestowed on the church as a teaching authority, as a gift of the Holy Spirit. It has been believed since the earliest times to be guaranteed in such scriptural passages as John 14.16,17. The analogous attribute of the Bible is usually called inerrancy. Protestants widely reject infallibility of the church. The Orthodox Eastern Church holds that only the church, taken as an integral community and spiritual body guided by the Holy Spirit, is infallible. Roman Catholics hold that the infallibility of the church is vested in the pope, when he speaks ex cathedra (i.e., from the chair of Peter, as the visible head of the church) on matters of faith and morals. Definitive pronouncements resulting from an ecumenical council, when ratified by the pope, are also held to be infallible. The pope speaks ex cathedra only rarely and after long deliberation. The dogma of papal infallibility was enunciated by the First Vatican Council (1870).

In Roman Catholicism, the doctrine that the pope, acting as supreme teacher and under certain conditions, as when he speaks ex cathedra (“from the chair”), cannot err when he teaches in matters of faith or morals. It is based on the belief that the church, entrusted with the teaching mission of Jesus, will be guided by the Holy Spirit in remaining faithful to that teaching. The First Vatican Council (1869–70) stated the conditions under which a pope may be said to have spoken infallibly: he must intend to demand irrevocable assent from the entire church in some aspect of faith or morals. The doctrine remains a major obstacle to ecumenical endeavours and is the subject of controversy even among Roman Catholic theologians.

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