The Magisterium is a religious body that plays heavily in Philip Pullman’s His Dark Materials fantasy novels. The group is roughly analogous to the Catholic Church in our world, though with some key differences.
In the history of the Church of Lyra’s world, things appear to have gone much the same as in our world, until the time of John Calvin, who in this alternate timeline became Pope. While in our world John Calvin was an important Protestant thinker, who based himself in Geneva, in Lyra’s world apparently there was no true Protestant Reformation, and instead the Church simply adopted many Protestant ideals.
Pope John Calvin moved the seat of the Church to Geneva, and began a process of decentralization. When he died, the Papacy itself was abolished, and the Magisterium became a much more bureaucratic, less centralized body. The Magisterium held a tight rein on society, perhaps as a result of having no turmoil between Protestant and Catholic divisions, and in the time of the books has retained control over most of society.
The Magisterium is the ultimate political, social, and religious body in these books. It dictates what can be taught, controls most of the world’s governments, and reacts brutally to dissent. The Magisterium is a distinctly religious organization, and bears many similarities to the Catholic Church during the time of the Inquisition.
Because of its decentralization, the Magisterium is also full of political infighting. Various factions vie for larger degrees of power, and are often played against one another. It is made apparent in the book that different groups within the Magisterium are withholding information from each other, in order to further their own political agendas. The three most powerful of these factions appear to be the General Oblation Board, the Society of the Work of the Holy Spirit, and the Consistorial Court of Discipline.
The General Oblation Board operates ex cathedra in many ways, and is seemingly responsible to neither the Consistorial Court of Disciple nor any other organization. Their primary task is to destroy Dust, which is viewed as a physical manifestation of Sin itself. In order to accomplish their goal, they are given free rein and have accrued immense power.
The Consistorial Court of Discipline is, as its name suggests, the disciplinary branch of the Magisterium. They investigate all claims of heresy, and have free rein to punish and kill as they see fit. The Court is a small, tight-knit group run by twelve fanatical priests, who see their work as keeping the Magisterium pure.
The Society of the Work of the Holy Spirit is a research branch of the Magisterium. Whereas the Consistorial Court of Discipline charges people with heresy and punishes them accordingly, the Society of the Work of the Holy Spirit acts to look into heresies and to place them into categories. They are essentially a scientific organization, although working within the strict confines of the Magisterium’s doctrine. They investigate the nature of Dust, and of the various other universes, for example, even though these things are considered heretical by the Magisterium itself.
The Magisterium is presented somewhat differently in the books and the movie adaptation of the first book, The Golden Compass. In the books it is made very clear that the Magisterium is simply an analog to the Catholic Church in our world, albeit a somewhat more vicious, more powerful Church. In the movie this connection is downplayed, and the Magisterium appears to be a more fictionalized body, perhaps to reduce the level of backlash from Church groups.