11 “In this know we love:” he means, perfection of love, that perfection which we have bidden you lay to heart: “In this know we love, that He laid down His life for us: and we ought to lay down our lives for the brethren.” Lo here, whence that came: “Peter, do you love me? Feed My sheep.” For, that you may know that He would have His sheep to be so fed by him, as that he should lay down his life for the sheep, straightway said He this to him: “When you were young, you girded yourself, and walked 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. This spoke He,” says the evangelist, “signifying by what death he should glorify God;” so that to whom He had said, Feed my sheep, the same He might teach to lay down his life for His sheep.
Source: Homilies on the First Epistle of John (New Advent)