TS 1: Infallibility
The Magisterium teaches infallibly in only three ways:
Sacred infallible Magisterium
1. Papal infallibility
The teaching of the First Vatican Council on Papal Infallibility can be conveniently summed up with a list of criteria that a teaching must meet in order to obtain "that infallibility with which the divine Redeemer willed His Church to be endowed…."
1. "the Roman Pontiff"
2. "speaks ex cathedra" ("that is, when in the discharge of his office as shepherd and teacher of all Christians, and by virtue of his supreme apostolic authority….")
3. "he defines"
4. "that a doctrine concerning faith or morals"
5. "must be held by the whole Church"
Whenever a teaching of the Pope by himself does not meet all of the above criteria, his teaching is not infallible. If anyone claims that the teaching of the Pope is always infallible, regardless of the above definition from the First Vatican Council, such a person has fallen into heresy.
2. The infallible definitions of the body of Bishops led by the Pope, which typically occurs in Ecumenical Councils
An Ecumenical Council occurs when the body of Bishops gather under the authority and leadership of the Pope, to exercise the spiritual and temporal authority of the Church. The group of Bishops must be representative of the universal Church, not merely the Bishops from one or a few nations. The Pope need not be personally present; he may send a representative. Only the teachings decisions of the Council approved by the Pope are valid and in force. Apart from the Pope, the Council has no authority. A Council cannot over-rule the current Pope, nor contradict past infallible teachings, nor teach with authority apart from the Pope. A Council cannot remove a Pope from office, nor take any action with the authority of the universal Church apart from the Pope. If a Pope dies during a Council, that Council ceases to be a Council; for Ecumenical Councils by definition must have the Pope as their head. A body without a head cannot continue.
Ecumenical Councils can teach infallibly, typically through Canons or Dogmatic Definitions. But a Council can also teach under the non-infallible Ordinary Magisterium. And a Council can exercise the temporal authority of the Church, as opposed to the teaching authority.
Sometimes the Pope may consult with the Bishops of the world by various means of communication, and then they may arrive at a definition of doctrine, which will have the authority of the definition of a Council, because it is the body of Bishops led by the Pope teaching definitively. Such definitive doctrinal decisions of the body of Bishops led by the Pope are infallible. (Three examples of this type of infallible definition of doctrine are found in the encyclical of Pope John Paul II, Evangelium Vitae.)
3. the Ordinary Universal Magisterium
"Although the individual bishops do not enjoy the prerogative of infallibility, they nevertheless proclaim Christ's doctrine infallibly whenever, even though dispersed through the world, but still maintaining the bond of communion among themselves and with the Successor of Peter, and authentically teaching matters of faith and morals, they are in agreement on one position as definitively to be held." Lumen Gentium, n. 25.
This is the most common way that the Church teaches infallibly. This type of infallible teaching has no one particular written definition. It occurs when successive Bishops and Popes, over the course of time, definitively teach one and the same doctrine on a question of faith or morals whose answer is found in Sacred Tradition or Sacred Scripture, while dispersed throughout the world. An example of such an infallible teaching would perhaps be the teaching of the Church against artificial birth control. Another example would perhaps be the teaching that Mary died and was raised from the dead prior to her Assumption.
It is sometimes difficult to discern which teachings fall under the Ordinary Universal Magisterium, since the Magisterium has not defined exact criteria for this type of teaching, and since there is no one written statement which constitutes the infallible teaching.
All other teachings fall under the Ordinary Magisterium, which is non-infallible. The teachings of the Ordinary Magisterium may contain errors on particular points, but never to such an extent as to lead the faithful away from the path of salvation.
The Difference between the Ordinary Magisterium and the Ordinary Universal Magisterium
The Ordinary Universal Magisterium is called 'ordinary' because such teachings generally begin under the non-infallible Ordinary Magisterium. And then, when the teaching has developed to the point that the Bishops and the Pope are in agreement on one position definitively to be taught, based on Sacred Tradition and Sacred Scripture, the teaching then falls under the Universal Magisterium and becomes an infallible teaching. Along the way from non-infallible to infallible, the teaching may undergo a development of doctrine, which prunes the teaching of any false or inaccurate ideas, adds all that is essential to a proper understanding of that particular point, and brings the teaching into full accord with the other teachings of the Church.