27 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 the necessity of belonging to the true Church in order to gain eternal salvation. Others finally belittle the reasonable character of the credibility of Christian faith.
Source: Humani Generis (Vatican.va)