From a sermon by Saint Augustine, bishop (Sermo 295, 1-2.4.7-8:PL 38, 1348-1352)
Saint Peter, the first of the apostles and a fervent lover of Christ, merited to hear these words: I say to you that you are Peter, for he had said: You are the Christ, the Son of the living God. Then Christ said: And I say to you that you are Peter, and on this rock I will build my Church. On this rock I will build the faith that you now confess, and on your words: You are the Christ, the Son of the living God, I will build my Church.
As you are aware, Jesus chose his disciples before his passion and called them apostles; and among these almost everywhere Peter alone deserved to represent the entire Church. And because of that role which he alone had, he merited to hear the words: To you I shall give the keys of the kingdom of heaven. For it was not one man who received the keys, but the entire Church considered as one. Now insofar as he represented the unity and universality of the Church, Peter’s preeminence is clear from the words: To you I give, for what was given was given to all.
Both apostles share the same feast day, for these two were one; and even though they suffered on different days, they were as one. Peter went first, and Paul followed. And so we celebrate this day made holy for us by the apostles’ blood. Let us embrace what they believed, their life, their labors, their sufferings, their preaching and their confession of faith.