The word “Magisterium,” coming from the Latin for “master” (“magister”), means in the Church either the Church’s authoritative teaching or its authorised teachers. Now as teacher is superior to taught, so the Magisterium teaching is superior to the Catholic people being taught. But the Catholic Masters have free-will, and God leaves them free to err. Then if they err gravely, may the people stand up to them and tell them, however respectfully, that they are wrong? The question is answered by truth. It is only when most people have lost the truth, as today, that the question can become confused.
On the one hand it is certain that Our Lord endowed his Church with a teaching authority, to teach us fallible human beings that Truth which alone can get us to Heaven – “Peter, confirm they brethren.” On the other hand Peter was only to confirm them in that faith which Our Lord had taught him – “I have prayed that thy faith fail not, and thou being converted, confirm thy brethren” (Lk. XXII, 32). In other words that faith governs Peter which it is his function only to guard and expound faithfully, such as it was deposited with him, the Deposit of Faith, to be handed down for ever as Tradition. Tradition teaches Peter, who teaches the people.
Vatican I (1870) says the same thing. Catholics must believe “all truths contained in the word of God or handed down by Tradition” and which the Church puts forward as divinely revealed, by its Extraordinary or Ordinary Universal Magisterium (one recalls that without Tradition in its broadest sense, there would have been no “word of God,” or Bible). Vatican I says moreover that this Magisterium is gifted with the Church’s infallibility, but this infallibility excludes any novelty being taught. Then Tradition in its broadest sense governs what the Magisterium can say it is, and while the Magisterium has authority to teach inside Tradition, it has no authority to teach the people anything outside of Tradition.
Yet souls do need a living Magisterium to teach them the truths of salvation inside Catholic Tradition. These truths do not change any more than God or his Church change, but the circumstances of the world in which the Church has to operate are changing all the time, and so according to the variety of these circumstances the Church needs living Masters to vary all the time the presentation and explanation of the unvarying truths. Therefore no Catholic in his right mind disputes the need for the Church’s living Masters.
But what if these Masters claim that something is inside Tradition which is not there? On the one hand they are learned men, authorised by the Church to teach the people, and the people are relatively ignorant. On the other hand there is for instance the famous case of the Council of Ephesus (428), where the people rose up in Constantinople to defend the divine Motherhood of the Blessed Virgin Mary against the heretical Patriarch Nestor.
The answer is that objective truth is above Masters and people alike, so that if the people have the truth on their side, they are superior to their Masters if the Masters do not have the truth. On the other hand if the people do not have the truth, thay have no right to rise up against the Masters. In brief, if they are right, they have the right. If they are not right, they have no right. And what tells if they are right or not? Neither Masters (necessarily), nor people (still less necessarily), but reality, even if Masters or people, or both, conspire to smother it.