Before Papal Infallibility Was Defined

The First Vatican Council defined the circumstances in which the pope speaks infallibly:

When, in the exercise of his office as shepherd and teacher of all Christians, in virtue of his supreme apostolic authority, he defines a doctrine concerning faith or morals to be held by the whole Church, he possesses, by the divine assistance promised to him in blessed Peter, that infallibility which the divine Redeemer willed his Church to enjoy in defining doctrine concerning faith or morals.

So the elements of papal infallibility are:

1. The pope is speaking as head of the Church.
2. The pope is defining a doctrine to be believed by all in the Church.
3. The doctrine is a matter of faith or morality.

The Church then simply applied this definition to past papal declarations and determined that the only time this had happened up to that point in history had been Pope Pius IX’s definition of the Immaculate Conception in 1854.