|Vatican II (from stjohncc.net)|
"The Magisterium of the Church: The task of giving an authentic interpretation of the Word of God, whether in its written form or in the form of Tradition, has been entrusted to the living, teaching office of the Church alone. Its authority in this matter is exercised in the name of Jesus Christ. This means that the task of interpretation has been entrusted to the bishops in communion with the successor of Peter, the Bishop of Rome... Mindful of Christ's words to his apostles: 'He who hears you, hears me,' the faithful receive with docility the teachings and directives that their pastors give them in different forms" (CCC 85-87).
"The Church's teaching authority, vested in the bishops, as successors of the Apostles, under the Roman Pontiff, as successor of St. Peter. Also vested in the Pope, as Vicar of Christ and visible head of the Catholic Church." ~Father John Hardon; Modern Catholic Dictionary
What is Infallibility?
Is the Magisterium Infallible?
- Matthew 28:18-20: "And Jesus coming, spoke to them, saying: All power is given to me in heaven and in earth. Going therefore, teach all nations: baptizing them in the name of the Father and of the Son and of the Holy Ghost. Teaching them to observe all things whatsoever I have commanded you. And behold I am with you all days, even to the consummation of the world." (DR)
- Matthew 16:18: "And I say to you: That you are Peter; and upon this rock I will build my church, and the gates of hell shall not prevail against it." (DR)
- John 14:16-17: "And I will pray the Father, and he will give you another Counselor, to be with you for ever, even the Spirit of truth, whom the world cannot receive ... you know him, for he dwells with you, and will be in you." (RSVCE)
- John 16:13-15: "When the Spirit of truth comes, he will guide you into all the truth; for he will not speak on his own authority, but whatever he hears he will speak, and he will declare to you the things that are to come. He will glorify me, for he will take what is mine and declare it to you. All that the Father has is mine; therefore I said that he will take what is mine and declare it to you." (RSVCE)
- 1 Timothy 3:14-15: "I hope to come to you soon, but I am writing these instructions to you so that, if I am delayed, you may know how one ought to behave in the household of God, which is the church of the living God, the pillar and bulwark of the truth." (RSVCE)
What are the Forms of the Magisterium?
Councils that are not ecumenical or recognized by the Pope: Certain gatherings of bishops, for example the United States Conference of Catholic Bishops, when they come to decisions or publish documents, do not carry the weight of infallibility. Popes are, however, able to recognize regional councils like the Second Council of Orange (531) and thereby make them infallible.
Certain occasions within the Ordinary Magisterium of bishops: Notice the qualifiers within Lumen Gentium 25 that were quoted above. The Ordinary Magisterium is NOT infallible if:
Papal congregations: The congregations within the Holy See, for example the Congregation of the Doctrine of the Faith or the Congregation for the Causes of Saints, are NOT infallible. Even if popes approve a congregation's decision, it does not prove it infallible unless the decision regards faith and morals AND that Pope intends to teach the decision ex cathedra. For example, in the case of Galileo Galilei, "the Congregation of the Index in 1616 decreed that Galileo was not to teach or defend in the future the Copernican theory as an established fact. Although approved by the pope, the decree was purely disciplinary" and therefore NOT infallible. 
- Extraordinary Magisterium: "The Church's teaching office exercised in a solemn way, as in formal declarations of the Pope or of ecumenical councils of bishops approved by the Pope." ~Fr. John Hardon; Modern Catholic Dictionary
- Ordinary Magisterium: "The teaching office of the hierarchy under the Pope, exercised normally, that is, through the regular means of instructing the faithful. These means are all the usual channels of communication, whether written, spoken, or practical." ~Fr. John Hardon; Modern Catholic Dictionary
When is the Magisterium Infallible?
There are several levels of authority that correspond to infallibility within the Church's teaching:
- Solemn Definition by Popes (Extraordinary): Solemn definitions are those delivered by the Pope and carry the weight of infallibility. Solemn definitions are spoken ex cathedra ("from the chair") by the Pope and are an exercise in his teaching primacy as the successor of Saint Peter. These include Pope Pius IX's solemn definition of the Immaculate Conception, Ineffibilis Deus, and Pope Pius XII's definition of the Virgin Mary's Assumption into Heaven, Munificentissimus Deus. Papal definitions do NOT have to be in communion with the rest of the bishops to carry the weight of infallibility.
"The Roman Pontiff, when he speaks ex cathedra—that is, when in discharge of the office of pastor and teacher of all Christians, by virtue of his supreme apostolic authority, he defines a doctrine regarding faith or morals to be held by the universal Church, by the divine assistance promised to him in Blessed Peter, is possessed of that infallibility with which the divine Redeemer willed that his Church should be endowed in defining doctrine regarding faith or morals; and therefore such definitions are irreformable of themselves, and not in virtue of consent of the Church." ~Vatican I
- Ecumenical Councils (Extraordinary): All 21 of the Church's Councils, from the First Council of Nicaea in 325 to Vatican II in 1965, carry the weight of infallibility because they are the living voice of Christ as it speaks through the whole body of bishops, always in communion with the Bishop of Rome, the Pope.
"The supreme power in the universal Church ... is exercised in a solemn way in an ecumenical council. A council is never ecumenical unless it is confirmed or at least accepted as such by the successor of Peter; and it is prerogative of the Roman Pontiff to convoke these councils, to preside over them and to confirm them." ~Vatican II; Lumen Gentium
- Daily Teaching of the Church (Ordinary): The everyday teaching of bishops throughout the world, when they are in collaboration with each other and are in communion with the Pope, is infallible when it is proposed definitively, as something that needs to be held by the faithful.
"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." ~Vatican II; Lumen Gentium
- Certain Occasions within Papal Encyclicals (Ordinary): Pius XII, in his encyclical Humani Generis, says "nor must it be thought that what is expounded in Encyclical Letters does not of itself demand consent, since in writing such Letters the Popes do not exercise the supreme power of their Teaching Authority. For these matters are taught with the ordinary teaching authority, of which it is true to say: "He who heareth you, heareth me"; and generally what is expounded and inculcated in Encyclical Letters already for other reasons appertains to Catholic doctrine. But if the Supreme Pontiffs in their official documents purposely pass judgment on a matter up to that time under dispute, it is obvious that that matter, according to the mind and will of the same Pontiffs, cannot be any longer considered a question open to discussion among theologians."
- We note, however, with Fr. William Most that "not everything in Encyclicals, and similar documents, is on this level--this is true only when the Popes expressly pass judgment on a previously debated matter" and that "since the Church scattered throughout the world can make a teaching infallible without defining (see item 3 above), then of course the Pope alone, who can speak for and reflect the faith of the whole Church, can do the same even in an Encyclical, under the conditions enumerated by Pius XII." 
When Is the Magisterium NOT Infallible?
- Certain Papal Statements: Notice the qualifiers within the First Vatican Council's decree on Papal Infallibility quoted in part 1 above. The Pope is NOT infallible if:
- The Pope is not teaching "ex cathedra--that is, when in discharge of the office of pastor and teacher of all Christians"--OR the Pope is not teaching definitively through the Ordinary Magisterium by way of his encyclicals (see Pius XII's statement above).
- The Pope is not teaching about faith and morals
Keep in mind, if the Pope is teaching ex cathedra, he must be speaking about faith and morals. If he is teaching about faith and morals, however, he does not necessarily have to be teaching ex cathedra.
In addition, the Catholic Encyclopedia states that "not everything in a conciliar or papal pronouncement, in which some doctrine is defined [infallibly], is to be treated as definitive and infallible. For example, in the lengthy Bull of Pius IX defining the Immaculate Conception the strictly definitive and infallible portion is comprised in a sentence or two; and the same is true in many cases in regard to conciliar decisions. The merely argumentative and justificatory statements embodied in definitive judgments, however true and authoritative they may be, are not covered by the guarantee of infallibility which attaches to the strictly definitive sentences — unless, indeed, their infallibility has been previously or subsequently established by an independent decision." 
- Bishops speak individually
- Bishops speak outside of communion with the Pope
- Bishops teach about something other than faith and morals
- Bishops are in disagreement about a position
- Bishops do not intend their teaching to be definitively held
Religious Submission of Mind and Will When the Church Speaks Fallibly:
Paragraph 25 of Lumen Gentium, Vatican II's document on the Church, states that "Bishops, teaching in communion with the Roman Pontiff, are to be respected by all as witnesses to divine and Catholic truth. In matters of faith and morals, the bishops speak in the name of Christ and the faithful are to accept their teaching and adhere to it with a religious assent. This religious submission of mind and will must be shown in a special way to the authentic magisterium of the Roman Pontiff, even when he is not speaking ex cathedra; that is, it must be shown in such a way that his supreme Magisterium is acknowledged with reverence, the judgments made by him are sincerely adhered to, according to his manifest mind and will. His mind and will in the matter may be known either from the character of the documents, from his frequent repetition of the same doctrine, or from his manner of speaking."
Again Father William Most explains: "If they (Pope or bishops) do not mean to make it definitive, then it does not come under the virtue of faith, or the promise of Christ, 'He who hears you hears me.' Rather, it is a matter of what ... Canon [752 of the New Code of Canon Law] and LG 25 call "religious submission of mind and of will." What does this require? Definitely, it forbids public contradiction of the teaching. But it also requires something in the mind, as the wording indicates. This cannot be the absolute assent which faith calls for - for since this teaching is, by definition, not definitive, we gather that it is not absolutely finally certain." We see then that, though we owe the submission of our minds and wills to the Magisterium of both Pope and bishops (only when they teach about faith and morals), these statements do not carry the weight of infallibility.
To summarize these somewhat confusing sets of circumstances, we can remember these three necessities:
- Teaching regards faith and morals
- Teaching is in communion with the Pope
- Teaching is meant to be definitively held by the faithful
If all three of these requirements are met, the teaching is infallible.
When does the Church intend her teaching to be definitively held? We can again let Father Most answer this question:
- For the Extraordinary Magisterium: "No special formula of words is required in order to define. Wording should be something solemn, and should make clear that the teaching is definitive. Councils in the past often used the form: 'Si quis dixerit. . . anathema sit." That is: "If someone shall say. . . . let him be anathema." But sometimes they used the formula for disciplinary matters, so that form alone does not prove. Further, they also could define in the capitula, the chapters. Thus Pius XII, in Divino afflante Spiritu (EB 538) spoke of such a passage of Vatican I (DS 3006 -- saying God is the author of Scripture) as a solemn definition." 
- For the Ordinary Magisterium: "To know whether the Church intends to teach infallibly on this second level, we notice both the language - no set form required - and the intention, which may be seen at times from the nature of the case, at times from the repetition of the doctrine on this second level."
 Fr. William Most; Hierarchy of Truths and Four Levels of Teaching
 Patrick Toner; Catholic Encyclopedia; Infallibility
 Fr. John Hardon; Christ to Catholicism