Augustine of Hippo

"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, lovest thou me? Feed My sheep." For, that ye 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 thou wast young, thou girdedst thyself, and walkedst whither thou wouldest: but when thou shalt be old, thou shalt stretch forth thy hands, and another shall gird thee, and carry thee whither thou wouldest not. This spake He," saith 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.

Whence beginneth charity, brethren? Attend a little: to what it is perfected, ye have heard; the very end of it, and the very measure of it is what the Lord hath put before us in the Gospel: "Greater love hath no man," saith He, "than that one lay down his life for his friends." Its perfection, therefore, He hath put before us in the Gospel, and here also it is its perfection that is put before us: but ye ask yourselves, and say to yourselves, When shall it be possible for us to have this charity? Do not too soon despair of thyself. Haply, it is born and is not yet perfect; nourish it, that it be not choked.