The doctrine defined by Vatican Council I may be briefly stated as follows: “The Pope is infallible when he speaks ex cathedra, i.e., when, as Pastor and Teacher of all Christians, he defines, in virtue of his supreme Apostolic authority, a doctrine concerning faith or morals to be held by the universal Church. He is said to “define” a doctrine, when he makes it clear that the doctrine must be believed with a firm, interior assent of faith. The doctrine must be concerned with faith or morals and must belong to the Deposit of Faith, i.e., it must be found in Scripture or Sacred Tradition.”
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