The Magisterium defined

As scholars such as the late great Dominican theologian, Yves Cardinal Congar, have noted, the term magisterium has such a long history and during the Middle Ages it referred to the teaching authority proper to theologians, i.e., those who by study and diligence have achieved some understanding of the truths of the faith and their relationship to truths that can be known without the light of faith.

But today this term has a very precise meaning, one given it by the Church herself in her understanding of herself as the pillar and ground of truth (see Tim 3:15) against which the gates of hell cannot prevail (Mt 16:18; Gal 1:8), and as the community to which Christ himself has entrusted his saving word and work. According to her own understanding of the term, the Church teaches that the magisterium is the authority to teach, in the name of Christ, the truths of Christian faith and life (morals) and all that is necessary and/or useful for the proclamation and defense of these truths (see Dei verbum, 8). This teaching authority is vested in the college of bishops under the headship of the chief bishop, the Roman Pontiff, the “concrete center of unity and head of the whole episcopate,” the successor of the Apostle Peter (see Vatican Council II, Lumen Gentium, 22; Vatican Council I, DS 3065-3074).
This magisterium, moreover, demands assent to its teachings by the faithful in virtue of the divine authority vested in it and not simply in virtue of the contents of the message it teaches (Vatican Council I, DS 3020). It has authority in teaching all the faithful in keeping with the inner constitution of the Church itself (Lumen gentium, 23-24). Its teaching, moreover, is an exercise of its pastoral office, its munus (a term much richer in connotation than our English “office,” connoting a privileged honor and mission), to care for the “souls” of all the faithful, i.e., to safeguard the divine life within them.