The answer to this oft-asked question can be difficult to explain, because the world tends to see things in worldly terms only. And so the world’s answer seems logical:

The pope is human

Humans err

Therefore, the pope errs

And that’s correct – all popes (past, present, future) err. But (and here’s the key), they don’t err when teaching a matter of faith and morals with the authority handed down to them from St. Peter, the first pope.

This bears repeating: when a pope, after much prayer and consultation with fellow* bishops from around the world, declares a matter regarding faith and morals to be believed, and promulgated through the Magisterium (the teaching authority) of the Catholic Church, he cannot err.

How can this be? Who says that popes get such things right every time? Jesus, that’s Who! Christ Himself promised that the Holy Spirit would guide His Catholic Church to all truth {cf Jn 16:13}, and that the gates of Hell would never prevail upon His Church {cf Mt 16:18}.

In modern times, two doctrines have thus been taught: the doctrine of Mary’s Immaculate Conception, and the doctrine of Mary’s bodily Assumption into Heaven.

And let’s not lose sight of what a blessing Papal Infallibility truly is to the Catholic Church – and thus to the world! “. . . [for the] Magisterium’s task [is] to preserve God’s people from deviations and defections[,] and to guarantee them the objective possibility of professing the true faith without error” so that “the People of God abides in the truth . . . {CCC 890}

*a pope is, in fact, the bishop of Rome, Italy