Peter then was true; or rather was Christ true in Peter? Now when the Lord Jesus Christ would, He abandoned Peter, and Peter was found a man; but when it so pleased the Lord Jesus Christ, He filled Peter, and Peter was found true. The Rock (Petra) made Peter true, for the Rock was Christ. And what did He announce to him, when he answered a third time that he loved Christ, and a third time the Lord commended His little sheep to Peter? He announced to him beforehand his suffering. When you were young, says He, you girded yourself, and wentest whither you would, but when you shall be old, you shall stretch forth your hands, and another shall gird you, and carry you whither you would not. The Evangelist has explained to us Christ's meaning. This spoke He, says he, signifying by what death he should glorify God; that is that he was crucified for Christ; for this is, You shall stretch forth your hands. Where now is that denier? Then after this the Lord Christ said, Follow Me. Not in the same sense as before, when he called the disciples. For then too He said, Follow Me; but then to instruction, now to a crown. Was he not afraid to be put to death when he denied Christ? He was afraid to suffer that which Christ suffered. But now he must be afraid no more. For he saw Him now Alive in the Flesh, whom he had seen hanging on the Tree. By His Resurrection Christ took away the fear of death; and forasmuch as He had taken away the fear of death, with good reason did He enquire of Peter's love. Fear had thrice denied, love thrice confessed. The threefoldness of denial, the forsaking of the Truth; the threefoldness of confession, the testimony of love.
Augustine, Sermon 97 on the New Testament