7 One may say, “Who is my neighbour?” Every man is your neighbour. Had we not all the same two parents? Animals of every species are neighbours one to the other, the dove to the dove, the leopard to the leopard, the asp to the asp, the sheep to the sheep, and is not man neighbour to man? Call to mind the ordering of the creation. God spoke, the waters brought forth swimming creatures, great whales, fish, birds, and such like things. Did all the birds come of one bird?
Did all vultures come of one vulture? Did all doves come of one dove? Did all snakes come of one snake? Or all gilt-heads of one gilt-head? or all sheep of one sheep? No, the earth assuredly brought forth all these kinds together. But when it came to man, the earth did not bring forth man. One father was made for us; not even two, father and mother: one father, I say, was made for us, not even two, father and mother; but out of the one father came the one mother; the one father came from none, but was made by God, and the one mother came out of him.
Mark then the nature of our race: we flowed out of one fountain; and because that one was turned to bitterness, we all became from a good, a wild olive tree. And so grace came also. One begot us unto sin and death, yet as one race, yet as neighbours one to another, yet as not merely like, but related to each other. There came One against one; against the one who scattered, One who gathers. Thus against the one who slays, is the One who makes alive. “For as in Adam all die, even so in Christ shall all be made alive.” Now as whosoever is born of the first, dies; so whosoever believes in Christ is made alive. Provided, that is, that he have “the wedding garment,” and be invited as one who is to remain, and not to be cast out.
Source: Sermons on Selected Lessons of the New Testament (New Advent)