“When He the Spirit of truth has come, He will teach you all the truth” (John, 16:13)
The words of today's Gospel were spoken to the apostles by Our Blessed Lord on the night before His death. The apostles were saddened and uncertain, and Christ wished to console them by assuring them that in the near future He would send the Holy Ghost to give them light and strength. This promise was fulfilled on Pentecost, when the Holy Spirit descended upon the apostles and disciples in the form of tongues of fire, giving them wisdom and courage so that they were enabled to go forth into the streets of Jerusalem to preach the Gospel without fear or hesitation.
This promise of Our Blessed Saviour was directed to the apostles as the official teachers of His Church; hence, it was intended also for their successors in this office, the Bishops of the Catholic Church and, in the first place, the Pope. In these words of the Son of God we find confirmation of the doctrine of our Holy Faith which asserts that the Holy Ghost will protect these teachers from error in their task of communicating Christ's message to the souls of men even to the end of time.
To many non-Catholics the claim of the Church to infallibility seems preposterous. But it should appear most reasonable to anyone who admits that Jesus Christ is true God, able to watch over the teaching office of His representatives and empowered to send the third person of the Holy Trinity to protect these teachers from leading the faithful astray by error.
For this reason Catholics believe that their Church is infallible. The prerogative of infallibility is possessed by the Bishops when they assemble in council with the Pope, and also when, under his jurisdiction and approval, they agree in teaching some doctrine of faith or morals in their respective dioceses. Above all, infallibility belongs to the Pope when he speaks ex cathedra that is, when, as head of the entire Church, using the fullness of his teaching power, he defines a doctrine of faith or morals to be held by all the faithful. When Catholics receive a doctrine taught in this manner by the official teachers of their Church they know that it has come to them under the protecting power of the Holy Ghost and is infallibly true.
Thank God that when you make an act of faith in the doctrines taught by the Catholic Church you have absolute assurance that God Himself has protected those who announced it from error. Often make acts of faith in the teachings of the Church.