On 29th June, 1896, Pope Leo XIII issued his Encyclical Letter Satis Cognitum on the Unity of the Church:
"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 hath 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," constituted 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 con firm his brethren.""