The Catholic Faith is based upon the teachings of Christ to the Apostles, and the teachings of the Apostles to all the peoples of the world. From there it has been handed down intact by each successive generation of the heirarchy and of Catholic parents.
The Catholic Faith is a Divine Revelation from Almighty God. It is proved by the miracles wrought by Christ and his sucessors, foremost among them Christ's Resurrection from the Dead, but not excluding even so trifling a matter as the annual liquification of the blood of St. Januarius (San Gennaro in Italian) in Naples or the Eucharistic Miracle of Lanciano. If it were defined by human men, we would be worshipping ourselves and our own opinon.