Pius XII’s Humani Generis from 1950 was the last major encyclical warning Catholics about the dangers facing the Church prior to Vatican II. In it, he denounced what he termed the “imprudent zeal for souls” that led some Catholics to question the necessity of belonging to the Catholic Church: “Some say they are not bound by the doctrine, explained in Our Encyclical Letter of a few years ago, and based on the Sources of Revelation, which teaches that the Mystical Body of Christ and the Roman Catholic Church are one and the same thing. Some reduce to a meaningless formula…