Gregory the Great — as excerpted in the Catena Aurea on John 10:11-13

“(Hom. xiv. in Evang.) And He adds what that goodness (forma bonitatis) is, for our imitation: The good Shepherd giveth His life for the sheep. He did what He bade, He set the example of what He commanded: He laid down His life for the sheep, that He might convert His body and blood in our Sacrament, and feed with His flesh the sheep He had redeemed. A path is shewn us wherein to walk, despising death; a stamp is applied to us, and we must submit to the impression. Our first duty is to spend our outward possessions upon the sheep; our last, if it be necessary, is to sacrifice our life for the same sheep. Whoso doth not give his substance to the sheep, how can he lay down his life for them?”