91 The Gospel of Matthew gives a clear outline of the pastoral mission of Peter in the Church: "Blessed are you, Simon Bar-Jona! For flesh and blood has not revealed this to you, but my Father who is in heaven. And I tell you, you are Peter, and on this rock I will build my Church and the powers of death shall not prevail against it. I will give you the keys of the kingdom of heaven, and whatever you bind on earth shall be bound in heaven, and whatever you loose on earth shall be loosed in heaven" (16:17-19). Luke makes clear that Christ urged Peter to strengthen his brethren, while at the same time reminding him of his own human weakness and need of conversion (cf. 22:31-32). It is just as though, against the backdrop of Peter's human weakness, it were made fully evident that his particular ministry in the Church derives altogether from grace. It is as though the Master especially concerned himself with Peter's conversion as a way of preparing him for the task he was about to give him in his Church, and for this reason was very strict with him. This same role of Peter, similarly linked with a realistic affirmation of his weakness, appears again in the Fourth Gospel: "Simon, son of John, do you love me more than these? ... Feed my sheep" (cf. Jn 21:15-19). It is also significant that according to the First Letter of Paul to the Corinthians the Risen Christ appears to Cephas and then to the Twelve (cf. 15:5). It is important to note how the weakness of Peter and of Paul clearly shows that the Church is founded upon the infinite power of grace (cf. Mt 16:17; 2 Cor 12:7-10). Peter, immediately after receiving his mission, is rebuked with unusual severity by Christ, who tells him: "You are a hindrance to me" ( Mt 16:23). How can we fail to see that the mercy which Peter needs is related to the ministry of that mercy which he is the first to experience? And yet, Peter will deny Jesus three times. The Gospel of John emphasizes that Peter receives the charge of shepherding the flock on the occasion of a threefold profession of love (cf. 21:15-17), which corresponds to his threefold denial (cf. 13:38). Luke, for his part, in the words of Christ already quoted, words which the early tradition will concentrate upon in order to clarify the mission of Peter, insists on the fact that he will have to "strengthen his brethren when he has turned again" (cf. 22:32).
Source: Ut Unum Sint (Vatican.va)