On July 18, the Church will mark the 150th anniversary of Pastor Aeternus, Vatican I’s dogmatic constitution on the Church of Christ, which, in addition to describing the meaning and power of papal primacy exercised by St. Peter and his successors, formally defined as divinely revealed the dogma of papal infallibility.

It put into words what the Church had long believed — that when the pope, by virtue of his authority as shepherd and teacher of all Christians, defines a doctrine about faith or morals to be held by the whole Church, he does so infallibly by means of the divine assistance of the Holy Spirit that Jesus promised to Peter, the apostles and the Church.

The original context of this ecumenical council’s infallible declaration on the infallibility of the pope was a response, first, to the growing relativism of the 19th century, both in terms of general epistemology as well as to the truths of faith. To clarify that the pope can teach something as infallibly true is to proclaim insistently that there is truth. It implicitly states that God is real; that Jesus Christ, the incarnate Son of God and Truth that sets us free, taught infallibly; and that he has given his own capacity to teach the truth without error to the Church and to those he has chosen, called and commissioned to lead the Church, who exercise it, not independent of him, but in communion.

Second, it was a response to some of the questions that had arisen after the clear exercise of solemn papal infallibility 16 years earlier in Blessed Pius IX’s dogmatic declaration of Mary’s Immaculate Conception, especially with regard to the limits of papal infallibility. Some ultramontanists — those who advocate supreme papal authority in matters of faith and discipline — had been arguing that papal infallibility extended to basically all statements of the pope, even informal ones, a position that not only confused many Catholics, especially those who knew papal history like St. John Henry Newman, but emboldened and ignited Protestants and rationalists who misunderstood papal infallibility, as well.

Pastor Aeternus specified the conditions under which the pope proclaims a truth of faith in a solemnly infallible way: when he teaches ex cathedra, with the authority of the “chair of St. Peter,” as a teacher for the Church in all places and times, on a point drawn from the deposit of faith (Scripture and Tradition) that every member of the Church must believe and accept concerning Christian faith and life.

It didn’t mean, to quote Rex Mottram in Evelyn Waugh’s Brideshead Revisited, that if the pope said it was raining cats and dogs, Catholics needed to believe spiritual pets were pouring down from heaven. Popes can be, and occasionally are, fallible in their informal teachings, theological and historical understandings and pastoral applications. If they were to be given a test on the Bible or the Catechism, they wouldn’t be guaranteed to get every question right. When he teaches extra cathedram, he is fallible like the rest of us, and as Pope Benedict humbly emphasized in his introduction to his personal trilogy Jesus of Nazareth, theologians, Christians and others are free to correct him when he does.

But Pastor Aeternus did specify that when the pope teaches under the specified conditions, he will be prevented by the Holy Spirit from teaching error. Most theologians believe the pope has exercised this “solemn Magisterium” only twice: in 1854, with the dogma clarifying that Mary from the moment of her conception was free of original sin; and in 1950, with Pius XII’s dogmatic proclamation that at the end of her earthly life Mary was assumed body and soul into heaven. Others will say that, by Pastor Aeternus’ criteria, Pope Leo exercised it in his Tome to Flavian in 449 that led to the declaration of the Council of Chalcedon; Pope Agatho did so in a letter in 680 about Christ’s two wills; Pope Benedict XII exercised it in 1336 on the beatific vision of the just before the general resurrection; and Innocent X in 1653 and Pius VI in 1794 did so, condemning various principles and teachings of the heresy of Jansenism. Regardless, the exercise of the “solemn Magisterium” has been rare.

These instances, however, are not the only times that the Church or the pope as head of the Church have taught infallibly. The early ecumenical councils taught infallibly about the consubstantial reality of the Trinity, the divinity of Christ, his two natures and human and divine wills, the divinity of the Holy Spirit and several other truths we proclaim in the Creed. Likewise the popes and subsequent councils have infallibly taught that the Church was willed and founded by Jesus; that the sacraments were instituted by Jesus and communicate efficaciously their respective graces; that Christ is really and substantially present in the Holy Eucharist; that Sacred Scripture is inerrant with regard to the truths of salvation; that the pope exercises primacy in the Church by the will of Christ; that only a baptized male validly receives priestly ordination; that a person’s spiritual soul is immortal and is judged immediately upon death; and that the direct and voluntary killing of an innocent human being is gravely immoral.  

The theological ground for this broader exercise of the infallibility given to the Church was clarified by the Second Vatican Council in its Dogmatic Constitution Lumen Gentium. It described that the infallibility Christ has given to his Church so that it may pass on the faith in all its purity can be, and has been, exercised not only by the pope in solemn ex cathedra decrees and not just by bishops united with the pope in dogmatic decrees of ecumenical councils, but also in the “ordinary and universal Magisterium,” when the successors of Peter and the apostles together teach something to be part of the deposit of divine Revelation — founded on the Word of God and constantly preserved and applied in the Tradition of the Church — that must be adhered to with the obedience of faith. This is the normal way the Church’s infallibility is exercised.

Lumen Gentium similarly specifies that “the entire body of the faithful, anointed as they are by the Holy One, cannot err in matters of belief … when from the Bishops down to the last faithful they show universal agreement in matters of faith and morals” (12), which is something seen with regard to many of the teachings in the above paragraph. The Holy Spirit, whom Jesus promised that he and the Father would send to guide us into all the truth, works in the Church to prevent the Church from erring about what we must believe and do to please God and enter into his life.

One of the concerns that was prudentially expressed about the dogma of infallibility 150 years ago was that if the pope and the Church were to begin to define things as infallible, it might undermine the authority of every other teaching. If a doctrine were not infallibly defined and therefore irreformable, some worried, it would seem up for grabs and people would feel free to believe and to do the opposite. This concern that the “infallible Magisterium” might unintentionally spawn a fictitious “fallible magisterium” has come to light over the last 150 years, and even more so in the last 50.

Some on the progressive side downplay, doubt, disobey and occasionally outright dismiss Church teaching on contraception, the ordination of women as priests and the condemnation of all extramarital sexual activity because it hasn’t been proclaimed ex cathedra. Some on the conservative side do similarly with regard to Church teaching on the application of the death penalty, the environment, the universal destination of goods and religious freedom. Both replace the concept of authority with that of infallibility as well as substitute the truth of a doctrine taught by the Church with the degree of formal certitude with which the Church teaches it. Both end up placing non-infallibly expressed teachings of the Church at the level of theological opinions that they deem they are free to disregard.

It’s for this reason that the Second Vatican Council clarified that even when the pope and the bishops teaching in communion with him propose a teaching on faith and morals in the exercise of their normal teaching office — without pronouncing it in an infallible or definitive way — we are to adhere it with “religious assent.” Our response is different from the “assent of faith” we give to the truths in the deposit of faith — in which we place our trust in God teaching us through the Church — but it is an extension of it.

As we mark the sesquicentennial of Pastor Aeternus, it is an occasion for us to be grateful for the gift of the teaching authority of the Church, especially that of the pope, so that through it, we can know without doubt the truth that leads to salvation, to believe it, to live it and to transmit it more effectively.