11 The nature of this supreme authority, which all Christians are bound to obey, can be ascertained only by finding out what was the evident and positive will of Christ. Certainly Christ is a King for ever; and though invisible, He continues unto the end of time to govern and guard His church from Heaven. But since He willed that His kingdom should be visible He was obliged, when He ascended into Heaven, to designate a vice-gerent on earth. "Should anyone say that Christ is the one head and the one shepherd, the one spouse of the one Church, he does not give an adequate reply. It is clear, indeed, that Christ is the author of grace in the Sacraments of the Church; it is Christ Himself who baptizes; it is He who forgives sins; it is He who is the true priest who bath offered Himself upon the altar of the cross, and it is by His power that His body is daily consecrated upon the altar; and still, because He was not to be visibly present to all the faithful, He made choice of ministers through whom the aforesaid Sacraments should be dispensed to the faithful as said above" (cap. 74). "For the same reason, therefore, because He was about to withdraw His visible presence from the Church, it was necessary that He should appoint someone in His place, to have the charge of the Universal Church. Hence before His Ascension He said to Peter: 'Feed my sheep' " (St. Thomas, Contra Gentiles , lib. iv., cap. 76). Jesus Christ, therefore, appointed Peter to be that head of the Church; and He also determined that the authority instituted in perpetuity for the salvation of all should be inherited by His successors, in whom the same permanent authority of Peter himself should continue. And so He made that remarkable promise to Peter and to no one else: "Thou are Peter, and upon this rock I will build my church" ( Matt . xvi., 18). "To Peter the Lord spoke: to one, therefore, that He might establish unity upon one" (S. Pacianus ad Sempronium, Ep. iii., n. 11). "Without any prelude He mentions St. Peter's name and that of his father (Blessed art thou Simon, son of John) and He does not wish Him to be called any more Simon; claiming him for Himself according to His divine authority He aptly names him Peter, from petra the rock, since upon him He was about to found His Church" (S. Cyrillus Alexandrinus, In Evang. Joan. , lib. ii., in cap. i., v. 42).
Source: Satis Cognitum (Vatican.va)