4 It cannot be doubted that duties more numerous and of greater moment devolve on Catholics than upon such as are either not sufficiently enlightened in relation to the Catholic faith, or who are entirely unacquainted with its doctrines. Considering that forthwith upon salvation being brought out for mankind, Jesus Christ laid upon His Apostles the injunction to "preach the Gospel to every creature," He imposed, it is evident, upon all men the duty of learning thoroughly and believing what they were taught. This duty is intimately bound up with the gaining of eternal salvation: "He that believeth and is baptized shall be saved; but he that believeth not, shall be condemned."(2) But the man who has embraced the Christian faith, as in duty bound, is by that very fact a subject of the Church as one of the children born of her, and becomes a member of that greatest and holiest body, which it is the special charge of the Roman Pontiff to rule with supreme power, under its invisible head, Jesus Christ.
Source: Sapientiae Christianae (Vatican.va)