12 From this text it is clear that by the will and command of God the Church rests upon St. Peter, just as a building rests on its foundation. Now the proper nature of a foundation is to be a principle of cohesion for the various parts of the building. It must be the necessary condition of stability and strength. Remove it and the whole building falls. It is consequently the office of St. Peter to support the Church, and to guard it in all its strength and indestructible unity. How could he fulfil this office without the power of commanding, forbidding, and judging, which is properly called jurisdiction ? It is only by this power of jurisdiction that nations and commonwealths are held together. A primacy of honour and the shadowy right of giving advice and admonition, which is called direction , could never secure to any society of men unity or strength. The words - and the gates of Hell shall not prevail against it - proclaim and establish the authority of which we speak. "What is the it ?" (writes Origen). "Is it the rock upon which Christ builds the Church or the Church? The expression indeed is ambiguous, as if the rock and the Church were one and the same. I indeed think that this is so, and that neither against the rock upon which Christ builds His Church nor against the Church shall the gates of Hell prevail" (Origenes, Comment. in Matt., tom. xii., n. ii). The meaning of this divine utterance is, that, notwithstanding the wiles and intrigues which they bring to bear against the Church, it can never be that the church committed to the care of Peter shall succumb or in any wise fail. "For the Church, as the edifice of Christ who has wisely built 'His house upon a rock,' cannot be conquered by the gates of Hell, which may prevail over any man who shall be off the rock and outside the Church, but shall be powerless against it" ( Ibid .). Therefore God confided His Church to Peter so that he might safely guard it with his unconquerable power. He invested him, therefore, with the needful authority; since the right to rule is absolutely required by him who has to guard human society really and effectively. This, furthermore, Christ gave: "To thee will I give the keys of the kingdom of Heaven." And He is clearly still speaking of the Church, which a short time before He had called His own , and which He declared He wished to build on Peter as a foundation. The Church is typified not only as an edifice but as a Kingdom , and every one knows that the keys constitute the usual sign of governing authority. Wherefore when Christ promised to give to Peter the keys of the Kingdom of Heaven, he promised to give him power and authority over the Church. "The Son committed to Peter the office of spreading the knowledge of His Father and Himself over the whole world. He who increased the Church in all the earth, and proclaimed it to be stronger than the heavens, gave to a mortal man all power in Heaven when He handed him the Keys" (S. Johannes Chrysostomus, Hom. liv., in Matt. v., 2). In this same sense He says: "Whatsoever thou shall bind upon earth it shall be bound also in Heaven, and whatsoever thou shalt loose on earth it shall be loosed also in Heaven." This metaphorical expression of binding and loosing indicates the power of making laws, of judging and of punishing; and the power is said to be of such amplitude and force that God will ratify whatever is decreed by it. Thus it is supreme and absolutely independent, so that, having no other power on earth as its superior, it embraces the whole Church and all things committed to the Church. The promise is carried out when Christ the Lord after His Resurrection, having thrice asked Peter whether he loved Him more than the rest, lays on him the injunction: "Feed my lambs - feed my sheep." That is He confides to him, without exception, all those who were to belong to His fold. "The Lord does not hesitate. He interrogates, not to learn but to teach. When He was about to ascend into Heaven He left us, as it were, a vice-gerent of His love....and so because Peter alone of all others professes his love he is preferred to all-that being the most perfect he should govern the more perfect" (S. Ambrosius, Exposit. in Evang. secundum Lucam , lib. x., nn. 175-176). These, then, are the duties of a shepherd: to place himself as leader at the head of his flock, to provide proper food for it, to ward off dangers, to guard against insidious foes, to defend it against violence: in a word to rule and govern it. Since therefore Peter has been placed as shepherd of the Christian flock he has received the power of governing all men for whose salvation Jesus Christ shed His blood. "Why has He shed His blood? To buy the sheep which He handed over to Peter and his successors" (S. Joannes Chrysostomus, De Sacerdotio , lib. ii). And since all Christians must be closely united in the communion of one immutable faith, Christ the Lord, in virtue of His prayers, obtained for Peter that in the fulfilment of his office he should never fall away from the faith. "But I have asked for thee that thy faith fail not" ( Luke xxii., 32), and He furthermore commanded him to impart light and strength to his brethren as often as the need should arise: "Confirm thy brethren" ( Ibid .). He willed then that he whom He had designated as the foundation of the Church should be the defence of its faith. "Could not Christ who confided to him the Kingdom by His own authority have strengthened the faith of one whom He designated a rock to show the foundation of the Church?" (S. Ambrosius, De Fide , lib. iv., n. 56). For this reason Jesus Christ willed that Peter should participate in certain names, signs of great things which properly belong to Himself alone: in order that identity of titles should show identity of power. So He who is Himself "the chief corner-stone in whom all the building being framed together, groweth up in a holy temple in the Lord" ( Eph . ii., 21), placed Peter as it were a stone to support the Church. "When he heard `thou art a rock,' he was ennobled by the announcement. Although he is a rock, not as Christ is a rock, but as Peter is a rock. For Christ is by His very being an immovable rock; Peter only through this rock. Christ imparts His gifts, and is not exhausted....He is a priest, and makes priests. He is a rock, and constitutes a rock" (Hom. de Poenitentia , n. 4 in Appendice opp. S. Basilii). He who is the King of His Church, "Who bath the key of David, who openeth and no man shutteth, who shutteth and no man openeth ( Apoc . iii., 7), having delivered the keys to Peter declared him Prince of the Christian commonwealth. So, too, He, the Great Shepherd, who calls Himself "the Good Shepherd," constitued Peter the pastor "of His lambs and sheep. Feed My lambs, feed My Sheep." Wherefore Chrysostom says: "He was preeminent among the Apostles: He was the mouthpiece of the Apostles and the head of the Apostolic College....at the same time showing him that henceforth he ought to have confidence, and as it were blotting out his denial, He commits to him the government of his brethren....He saith to him: 'If thou lovest Me, be over my brethren.' Finally He who confirms in "every good work and word" (2 Thess . ii., 16) commands Peter "to confirm his brethren." Rightly, therefore, does St. Leo the Great say: "From the whole world Peter alone is chosen to take the lead in calling all nations, to be the head of all the Apostles and of all the Fathers of the Church. So that, although in the people of God there are many priests and many pastors Peter should by right rule all of those over whom Christ Himself is the chief ruler" (Sermo iv., cap. 2). And so St. Gregory the great, writing to the Emperor Maurice Augustus, says: "It is evident to all who know the gospel that the charge of the whole Church was committed to St. Peter, the Apostle and Prince of all the Apostles, by the word of the Lord....Behold! he hath received the keys of the heavenly kingdom-the power of binding and loosing is conferred upon him: the care of the whole government of the Church is confided to him" (Epist. lib. v., Epist. xx).
Source: Satis Cognitum (Vatican.va)