Why do we claim any sort of teaching authority at all? It seems rather presumptuous to think that we human beings can teach with authority about spiritual matters. Remember that this was shocking to some of Jesus' own listeners, when they observed that "this man teaches with authority." Magisterium is founded on the promise of Christ that the Holy Spirit will remain present and active in the community of disciples. Consider John 14:26: "The Advocate, the holy Spirit that the Father will send in my name -- he will teach you everything and remind you of all that I told you." It is the Holy Spirit who teaches, no human authority.
The Holy Spirit is given to everyone at Baptism, and indeed, through all of the sacramental life of the Church. In the Middle Ages a distinction was introduced between the ecclesia docens (the teaching Church) and the ecclesia discens (the learning Church). However, of course, all of us participate, in a variety of degrees, in the one and the same teaching-and-learning Church. The role of the ordained is often highlighted. Vatican II spoke of the teaching role of bishops and presbyters as their primary obligation (the primum officium); subsequent documents have extended this as well to the renewed (permanent) diaconate as well. While one can sometimes read (usually in very derogatory terms) that others are claiming magisterial authority as well (consider some recent nasty blog posts about "the magisterium of the nuns" when criticizing the LCWR, for example), the medieval Church DID extend its understanding of magisterium beyond the bishops themselves. No less a theological giant than Thomas Aquinas in the 13th Century, wrote of the magisterium of university theologians; at the great Council of Trent in the 16th Century, and at Councils before that, the theological experts were often given a vote on conciliar texts! Trent, for example, referred to the theologians as minor theologians and the bishops as major theologians. Some votes included them all; some were restricted to the bishops alone.
UPDATE: After my initial post, I was doing some research on Blessed John XXIII's opening address o the world's bishops at the Second Vatican Council. I was struck by how he used the term magisterium, clearly using it the term accurately, and in the way we all used to use it until relatively recently. Look at how he introduces the term at the beginning of his address:
In calling this vast assembly of bishops, the latest and humble successor to the Prince of the Apostles who is addressing you intended to assert once again the magisterium, which is unfailing and perdures until the end of time, in order that this magisterium, taking into account the errors, requirements and opportunities of our time, might be presented in exceptional form to all people throughout the world.
Clearly, the pope is not referring to the college of bishops, with pope at the head of that college. Here's another wonderful example. The pope turns his attention to the previous 20 ecumenical (general) councils of the Church:
Finally, consider this famous passage:Ecumenical Councils, whenever they are assembled, are a solemn celebration of the union of Christ and His Church, and hence lead to the universal radiation of truth, to the proper guidance of individuals in domestic and social life, to the strengthening of spiritual energies for a perennial uplift toward real and everlasting goodness. The testimony of this extraordinary magisterium of the Church in the succeeding epochs of these twenty centuries of Christian history stands before us collected in numerous and imposing volumes, which are the sacred patrimony of our ecclesiastical archives, here in Rome and in the more noted libraries of the entire world.
"Good Pope John" (as the Italians still refer to him) shows us the proper understanding of this very complex term.The substance of the ancient doctrine of the deposit of faith is one thing, and the way in which it is presented is another. And it is the latter that must be taken into great consideration with patience if necessary, everything being measured in the forms and proportions of a magisterium which is predominantly pastoral in character.
We are all expected to "follow the magisterium of the Church." But this means so much more than simple obedience to the institutional structure of the external Church! After all, bishops alone are not given full knowledge of all religious Truth simply by virtue of their ordination as bishops! Like everyone else, they too grow "in wisdom, age and grace." "Following the magisterium" is not some kind of loyalty test; it means being attuned to the full "teaching office" found within the entire People of God, great and small.