In the American hymn "Battle Hymn of the Republic", to what does "the grapes of wrath" refer?

And another angel came out from the altar, who had power over fire, and he cried with a loud cry to him who had the sharp sickle, saying, "Thrust in your sharp sickle and gather the clusters of the vine of the earth, for her grapes are fully ripe." So the angel thrust his sickle into the earth and gathered the vine of the earth, and threw it into the great winepress of the wrath of God.