Augustine on John 15:
This passage of the Gospel, brethren, where the Lord calls Himself the vine, and His disciples the branches, declares in so many words that the Mediator between God and men, the man Christ Jesus, is the head of the Church, and that we are His members. For as the vine and its branches are of one nature, therefore, His own nature as God being different from ours, He became man, that in Him human nature might be the vine, and we who also are men might become branches thereof. What mean, then, the words, “I am the true vine”? Was it to the literal vine, from which that metaphor was drawn, that He intended to point them by the addition of “true”? For it is by similitude, and not by any personal propriety, that He is thus called a vine; just as He is also termed a sheep, a lamb, a lion, a rock, a corner-stone, and other names of a like kind, which are themselves rather the true ones, from which these are drawn as similitudes, not as realities. But when He says, “I am the true vine,” it is to distinguish Himself, doubtless, from that to which the words are addressed: “How art thou turned into sourness, as a strange vine?” For how could that be a true vine which was expected to bring forth grapes and brought forth thorns?
from “Homily on John 15,” by Augustine (c.354-430)