What is papal infallibility?
The Keys of St. Peter. (photo: Fr. Lawrence OP from Flickr via ChurchPop, CC BY NC ND 2.0))
Catechism of the Catholic Church
891 – “The Roman Pontiff, head of the college of bishops, enjoys this infallibility in virtue of his office, when, as supreme pastor and teacher of all the faithful – who confirms his brethren in the faith he proclaims by a definitive act a doctrine pertaining to faith or morals.
By Jeffrey Mirus, taken from ewtn.com
Since the successors of Peter have the same Petrine authority, which comes ultimately from Christ, to bind and loose, they have the authority to bind the faithful in matters pertaining to salvation—that is, in faith or morals. Now, if a Pope could bind the faithful to error, it would be a clear triumph of the powers of Hell, because the entire Church would be bound to follow the error under Christ’s own authority. Obviously, this cannot happen.
Therefore, the logic of the situation demands that the Petrine power of confirming the brethren must be an infallible power. When the Pope intends by virtue of his supreme authority to teach on a matter of faith and morals to the entire Church, he must be protected by the Holy Spirit from error—else the powers of hell would prevail.
Further, it is not a new thing. It was precisely defined at Vatican I in order to clarify what was at that time a confusing issue, but this was by way of stating clearly what Christ’s teaching was, not by way of adding anything new. Vatican I therefore carefully enumerated the conditions under which the Pope was in fact infallible—the same conditions which logic demands, which Scripture suggests, and which tradition shows us in action down through the centuries.
When the Pope (1) intends to teach (2) by virtue of his supreme authority (3) on a matter of faith and morals (4) to the whole Church, he is preserved by the Holy Spirit from error. His teaching act is therefore called “infallible” and the teaching which he articulates is termed “irreformable”.