This was a landmark in the teaching on Papal Infallibility. Previously, this truth was believed but never before stated in such a direct and concise manner. Examining history, we can see that it is possible for faith filled believers to err on Catholic Faith. St. Thomas Aquinas Is known as one of the greatest thinkers in the Church, however he did not believe in the Immaculate Conception of the Blessed Virgin Mary. The some is true for St. Catherine of Siena who Is a Doctor of the Church.
Good people can make errors.
This is why Christ left His Church a guide. He left His Church a gift. Infallibility Is not for the Holy Father, but for the good of the whole Church . That Christ gave primacy to one of His apostles, namely, Peter, and that he is empowered by Christ to teach the Church without error has been believed by the Church since the Lord founded Her. Throughout history, there are very few instances of the extraordinary use of this gift.