The Church is one because it is unified in belief.

The truths which the members of the Catholic Church hold are the truths made known to us by Jesus Himself; they are truths which come to us directly from God. God is Truth. He knows all things and cannot be mistaken. He is infinitely truthful and cannot lie. Whatever God has said is true forever and for everybody. It is not for us to pick and choose and to adjust God’s revelation to our own convenience. In the Catholic Church, all are obligated to believe the same truths. Thus, every Catholic must mean exactly the same thing when he recites the Apostles’ and Nicene Creeds.