Q. Faith and morals are the two areas in which the pope can make infallible declarations, that is as pope, as successor of St. Peter, the vicar of Christ on earth. I have always accepted this, and still do. But I draw a blank when I ask myself: What are the infallible papal declarations on morality? Nothing comes to mind, not even slavery. —Gerald

A. The Catholic Church is infallible when she proclaims matters pertaining to Divine Revelation. This proclamation can take four forms, two extraordinary and two ordinary. The extraordinary forms include proclamations by a pope ex cathedra (literally “from the chair”) and solemn doctrinal definitions of an ecumenical council; the ordinary forms include the infallibility of the ordinary and universal magisterium and the infallibility of the whole Christian faithful as expressed through “the supernatural appreciation of faith (sensus fidei) on the part of the whole people, when, from the bishops to the last of the faithful, they manifest a universal consent in matters of faith and morals" (CCC 92, quoting Lumen Gentium).

You asked about ex cathedra papal proclamations only. So I will reply to this first. But then I will reply to the broader question of what moral doctrines have been taught by other modes of infallibility.

That the successor of Peter, by the will of Christ, speaks on behalf of the infallible Church has been held and taught since the patristic period. But the formal doctrine of papal infallibility was not defined until the First Vatican Council (1870), where the Council fathers proclaimed:

“We teach and define as a divinely revealed dogma that when the Roman pontiff speaks ex cathedra … he possesses, by the divine assistance promised to him in blessed Peter, that infallibility which the divine Redeemer willed his church to enjoy in defining doctrine concerning faith or morals” (Pastor Aeternus).

The same Pope who promulgated Vatican I’s declaration explicitly invoked infallibility 16 years earlier when he solemnly defined the doctrine of the Immaculate Conception. His formulation began: “By the authority of Jesus Christ our Lord, of the Blessed Apostles Peter and Paul, and by our own: We declare, pronounce, and define that the doctrine which holds that the most Blessed Virgin Mary, in the first instance of her conception …” (Pope Pius IX, Ineffabilis Deus, Dec. 8, 1854; emphasis added). 

When, nearly 100 years later, Pope Pius XII defined the doctrine of the Assumption, he used the same intro formula: “By the authority of our Lord Jesus Christ, of the Blessed Apostles Peter and Paul, and by our own authority, we pronounce, declare, and define it to be a divinely revealed dogma: that the Immaculate Mother of God …” (Munificentissimus Deus, Nov. 1, 1950).

In these two instances, the papal intention to invoke infallibility was transparent to the whole Church. 

When Pope John Paul II authoritatively condemned abortion in 1995, he used a form not too different from the two Piuses, leading some, including myself, to wonder whether he intended to invoke infallibility. He began: “Therefore, by the authority which Christ conferred upon Peter and his Successors, in communion with the Bishops … I declare that direct abortion …” (Evangelium Vitae, 62). When consulted, the Vatican denied the pope had spoken ex cathedra. 

John Paul II could have invoked infallibility; and he certainly thought the wrongness of abortion was a de fide moral truth. So why didn’t he? He didn’t because he judged that since the Church has condemned abortion with unbroken continuity since the apostolic period, the conditions for an infallible teaching of the ordinary and universal magisterium had already been met. 

What are those conditions? Let us look at what the Second Vatican Council’s dogmatic constitution on the Church (Lumen Gentium) teaches about this mode of infallibility:

“Although the individual bishops do not enjoy the prerogative of infallibility, they nevertheless proclaim Christ’s doctrine infallibly whenever, even though dispersed throughout the world, but maintaining the bond of communion among themselves and with the successor of Peter, and authentically teaching matters of faith and morals, they agree about a judgment as one that is to be definitively held” (Lumen Gentium, 25).  

We see here four conditions set forth by the Council: 

1. the bishops though dispersed remain in union with the pope and one another;
2. they teach authentically on a matter of faith or morals, i.e., they teach as bishops;
3. they agree on a judgment;
4. they teach that judgment as to be definitively held (“definitive tenendam”).

When the four conditions are met, the teaching of the corresponding truth is guarded from error by the Holy Spirit.

Now let us consider the complete formulation of John Paul II’s condemnation of abortion: 

“Therefore, by the authority which Christ conferred upon Peter and his Successors, in communion with the Bishops — who on various occasions have condemned abortion and who in the aforementioned consultation, albeit dispersed throughout the world, have shown unanimous agreement concerning this doctrine — I declare that direct abortion, that is, abortion willed as an end or as a means, always constitutes a grave moral disorder, since it is the deliberate killing of an innocent human being. This doctrine is based upon the natural law and upon the written Word of God, is transmitted by the Church's Tradition and taught by the ordinary and universal Magisterium (Lumen Gentium 25).”

If we look closely, we can see the pope carefully tracking the words of Vatican II’s definition of the infallibility of the ordinary and universal magisterium: first, he makes clear he is teaching on a matter of faith and morals and that that teaching has been proclaimed by the bishops of the world, though dispersed yet remaining in communion with himself and each other; next, he affirms their unanimous agreement on the judgement against abortion; and finally, he declares that the truth he teaches is based on the natural law and Divine Revelation. By affirming the latter, he obviously means that his teaching is to be held precisely as such a truth, which is what definitive tenendam means. He then footnotes the exact paragraph from Lumen Gentium that I quoted above. 

The formulation reveals that John Paul II meant both to condemn abortion and to affirm that the truth contained in his condemnation is an infallible teaching of the ordinary and universal magisterium of the Catholic Church.

Admittedly, this mode of infallibility is not well understood. Unfortunately, its relevance for securing the dogmatic status of the Church’s moral doctrines is rejected by most dissenting moral theologians and given little attention by the wider theological community.

The wrongness of abortion is not the only moral doctrine taught by this mode of infallibility. Several others were elucidated by the Congregation for the Doctrine of the Faith in 1998 — e.g., the illicitness of euthanasia, prostitution and fornication (CDF, “Doctrinal Commentary on Professio fidei,” 11).

Are there others? A strong argument can be made that all the absolute negative norms in the area of sexual ethics that have been held and taught since the apostolic period meet the conditions for an infallible exercise of the ordinary and universal magisterium; additionally, all condemnations of acts that violate the unity-procreation paradigm affirmed in Humanae Vitae; and finally, all newer expressions of Fifth Commandment violations. Actions that fall under these three descriptions include the wrongness of all forms of non-marital sexual behavior such as homosexual, contraceptive and masturbatory acts, the bringing of children into the world outside the context of marital intercourse, and destructive and exploitative experimentation on embryonic human life. 

Ecumenical councils too can invoke infallibility. Perhaps the most important conciliar moral definition for us today is the Council of Trent’s infallible affirmation of the absolute indissolubility of a consummated Christian marriage and the council’s corresponding condemnation of divorce and remarriage as tantamount to adultery (see Trent’s “Canons on Marriage,” 5 and 7; see also E.C. Brugger, “The Indissolubility of Marriage and the Council of Trent”).

Finally, although in some cases we can readily identify when a pope invokes infallibility (e.g., the dogmas of the Immaculate Conception and the Assumption), in others cases it’s not so clear. Many bulls, decrees and professions of faith have been published by popes over the centuries proclaiming moral teachings on topics such as torture, Simony, usury, dueling, slavery, killing of tyrants, indissolubility of marriage, the wrongness of divorce and remarriage, capital punishment and original sin. As to whether these proclamations entailed the invocation of infallibility is a judgment that the Church must ultimately answer with the assistance of the best theological scholarship. 

Since Vatican II, Rome has been reticent to speak much about infallibility. This is likely because of a concern that too much talk about it would have the unintended side-effect of people seeing the corresponding doctrines legalistically — that is, as exercises of authority imposing extrinsic rules, and not as the handing on moral truths necessary for human flourishing. 

But Rome will soon need to rectify this for the sake of the spiritual health of the Catholic faithful, especially in the light of confusions regarding certain moral doctrines that have arisen in recent years. 

(For more on the infallibility of the ordinary and universal Magisterium, see the CDF’s “Doctrinal Commentary on the Concluding Formula of the Professio fidei,” 9. Note: you’ll need to scroll down to the fourth document on the webpage in this link.)