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 (186970) 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.