In this excerpt from a sermon on the Lord’s Supper delivered by Augustine of Hippo to a group of Catechumens, (a Christian believer preparing for Baptism) the great bishop compares the process in which a seed becomes wheat, which ultimately becomes bread, and then Communion, to the process of becoming a baptized Christian. Augustine, following in the footsteps of Jesus, likens the process of sanctification to the threshing of wheat, with the separation of the wheat from the chaff.
Call to mind what this created thing [bread] once was in the field. How the earth brought it forth, the rain nourished it, and ripened it into an ear of wheat and then human labor brought it together on the threshing floor, threshed it, winnowed it, stored it up again, took it out, ground it, added water to it, baked it, and only at that moment made it into the form of a loaf.
Call to mind also: you did not exist, you were created, you were brought together to the threshing floor of the Lord by the labor of the oxen, that is, by those who announced the gospel, by their work you have been threshed.
When as catechumens you had to wait [for your baptism], you were stored up in the granary. You had given your names [put them on a list for baptism], and you began to be ground by fasting and exorcisms. Later on you came to the water, and you were sprinkled, and you were made one. When the fervor of the Holy Spirit came upon you, you were baked and you were made into the loaf of the Lord.
See what you have received. Just as, therefore, you see that the loaf which has been made is one, so you also are to be one, by loving one another, by keeping one faith, one hope, and undivided love.
Taken from Augustine of Hippo, Third Sermon: Sermon Denis 6, 1–3.