Catholic apologists have long uttered the phrase, Roma locuta est, causa finita est. When Rome has spoken, the matter is finished. When conciliarists tried to subvert papal teaching in the 19th century, Pope Pius IX reiterated the divinely revealed dogma that whenever the Pope speaks definitively as teacher of all Christians ex cathedra and defines a doctrine concerning faith or morals, he possesses infallibility. These infallible teachings must be believed with divine and Catholic faith. Any Catholic who opposes them would fall into the sin — and canonical crime — of heresy. But these ex cathedra expressions of papal infallibility have only happened twice in the history of the Church.
Typically, popes exercise their teaching authority through their “ordinary magisterium,” or when they teach authentically, but not through a definitive infallible act, such as through an encyclical. This teaching is important. In fact, it requires religious respect of intellect and will by the faithful. But it’s not the same thing as ex cathedra infallible teaching. Even the pope’s ordinary magisterium, however, is distinguished from his personal opinion. When Joseph Ratzinger wrote the scriptural commentary Jesus of Nazareth while pope, he explained that the text was attributable to him personally, not to his role as Vicar of Christ.
The contents of the text may be true, and perhaps even helpful; but they lack the same weight as the authentic papal magisterium. Thus, when the media sloppily reported on recent comments by Pope Francis regarding same-sex civil unions — which may have been mistranslated — they falsely attributed private comments to the authentic papal magisterium. The Church has never taught that everything a pope says is infallible. A pope’s personal comment can’t change the immutable moral teachings of the Church. On the matter of unchaste romantic relationships, Rome has already spoken. The matter is finished.