The Church teaches infallibly when it defines, through the Pope alone, as the teacher of all Christians, or through the Pope and the bishops, a doctrine of faith or morals to be held by all the faithful.

The Church, as the representative or substitute of Jesus Christ on earth, is infallible, and speaks with His own words: "This is why I was born, and why I have come into the world, to bear witness to the truth" (John 18:37).

When the Church makes an infallible pronouncement, we are not to suppose that a new doctrine is being introduced. For instance, when the Holy Father in 1854 defined the Blessed Virgin's Immaculate Conception as an article of faith, the infallible definition was not a proclamation of a new doctrine, but was merely an announcement of an article of faith true from the very beginning, and publicly defined only in order to make the dogma clear to all and to be believed as part of the deposit of faith left to the Church.

Another example is the definition of the Holy Father's infallibility, made in 1870 by the Vatican Council. The dogma was true from the very beginning, and had been universally held. But as in recent times many objections were being made against it, the Bishops in the Vatican Council thought it best, in order to make clear the stand of the Church, to make an infallible definition.

The Church makes infallible pronouncements on doctrines of faith and morals, on their interpretation, on the Bible and Tradition, and the interpretation of any part or parts of these. The dogma of the Inmaculate Conception of the Blessed Virgin was an interpretation of a long-standing Tradition in the Church.

 
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