4 Let us, looking at ourselves in this member of the Church, distinguish what is of God, and what of ourselves. For then we shall not totter, then shall we be founded on the Rock, shall be fixed and firm against the winds, and storms, and streams, the temptations, I mean, of this present world. Yet see this Peter, who was then our figure; now he trusts, and now he totters; now he confesses the Undying, and now he fears lest He should die. Wherefore? Because the Church of Christ has both strong and weak ones; and cannot be without either strong or weak; whence the Apostle Paul says, “Now we that are strong ought to bear the infirmities of the weak.” In that Peter said, “You are the Christ, the Son of the living God,” he represents the strong: but in that he totters, and would not that Christ should suffer, in fearing death for Him, and not acknowledging the Life, he represents the weak ones of the Church. In that one Apostle then, that is, Peter, in the order of Apostles first and chiefest, in whom the Church was figured, both sorts were to be represented, that is, both the strong and weak; because the Church does not exist without them both.
Source: Sermons on Selected Lessons of the New Testament (New Advent)