On Dec. 8, 1869, the feast of Mary’s Immaculate Conception in the Catholic calendar, the resoundingly conservative Pope Pius IX opened the First Vatican Council. The following July 18, Pius and the gathering of global bishops climaxed a long-running church struggle and issued a constitution that defines papal authority in the strongest terms.

The upcoming 150th anniversary is a good moment for writers to tap ecumenists, historians, and theologians and ask them point blank: Is it a pipe dream to suppose the world’s Christians will someday, somehow reunite within a single fellowship?

The Second Vatican Council raised hopes with Unitatis Redintegrtio, its 1964 decree on ecumenism. The result is respectful talks and far friendlier relationships among separated churches, but perhaps that’s the best one can hope for. If so, Vatican I is the chief reason. Will your sources will think that’s an overstatement?

Those who’ve heard something about Vatican I will know it proclaimed that the pope is infallible. However, a pope exercises divinely given infallibility only when he defines “a doctrine concerning faith or morals to be held by the whole church” and “speaks ex cathedra.” That Latin phrase, literally “from the chair,” refers to formal exercise of his office’s authority. More broadly, Vatican II declared that Catholics must show “submission of will and of mind” to any authentic teachings by a pope.

In the usual understanding, infallibility applies only when a pope specifies this. This action has been very, very rare, with only one case since Vatican I. See this summary from the U.S. Catholic website:

There is no set list of ex cathedra teachings, but that’s because there are only two, and both are about Mary: her Immaculate Conception (declared by Pope Pius IX in 1854 and grandfathered in after the First Vatican Council’s declaration of papal infallibility in 1870) and her bodily Assumption into heaven (declared by Pope Pius XII in 1950).

But neither of these was earth-shattering to Roman Catholics, because these beliefs had been nurtured through devotion, prayer, and local teaching for centuries before becoming official papal teaching.

Importantly, Vatican I stated that such papal teachings are, and “not by the consent of the church, irreformable,” and pronounced an “anathema” against anyone who dissents.