"(R)eligiously guard and faithfully expound the revelation or deposit of faith."

The Apostle Peter by Peter Paul Rubens

The Holy Spirit was promised to the successors of Peter not so that they might, by his revelation, make known some new doctrine, but that, by his assistance, they might religiously guard and faithfully expound the revelation or deposit of faith transmitted by the apostles. Indeed, their apostolic teaching was embraced by all the venerable fathers and reverenced and followed by all the holy orthodox doctors, for they knew very well that this See of St. Peter always remains unblemished by any error, in accordance with the divine promise of our Lord and Savior to the prince of his disciples: "I have prayed for you that your faith may not fail; and when you have turned again, strengthen your brethren."—Pastor aeternus

Neque enim Petri successoribus Spiritus Sanctus promissus est, ut eo revelante novam doctrinam patefacerent, sed ut eo assistente traditam per Apostolos revelationem seu fidei depositum sancte custodirent et fideliter exponerent. Quorum quidem apostolicam doctrinam omnes venerabiles Patres amplexi et sancti Doctores orthodoxi venerati atque secuti sunt; plenissime scientes, hanc sancti Petri Sedem ab omni semper errore illibatam permanere, secundum Domini Salvatoris nostri divinam pollicitationem discipulorum suorum principi factam: Ego rogavi pro te, ut non deficiat fides tua, et tu aliquando conversus confirma fratres tuos.

To paraphrase the Christmas homily of Pope Saint Leo the Great: Remember, Bishop of Rome, your dignity.