“I am not unmindful of the promise by which I pledged myself to deliver a sermon to instruct you, who have just been baptized, on the Sacrament of the Lord’s table, which you now look upon and of which you partook last night. You ought to know what you have received, what you are going to receive, and what you ought to receive daily. That Bread which you see on the altar, consecrated by the word of God, is the Body of Christ. That chalice, or rather, what the chalice holds, consecrated by the word of God, is the Blood of Christ. Through those accidents the Lord wished to entrust to us His Body and the Blood which He poured out for the remission of sins. If you have received worthily, you are what you have received, for the Apostle says: ‘The bread is one; we though many, are one body?” Thus he explained the Sacrament of the Lord’s table: ‘The bread is one; we though many, are one body.’ So, by bread you are instructed as to how you ought to cherish unity.
“Was that bread made of one grain of wheat? Were there not, rather, many grains? However, before they became bread, these grains were separate; they were joined together in water after a certain amount of crushing. For, unless the grain is ground and moistened with water, it cannot arrive at that form which is called bread. So, too, you were previously ground, as it were, by the humiliation of your fasting and by the sacrament of exorcism. Then came the baptism of water; you were moistened, as it were, so as to arrive at the form of bread.
“But, without fire, bread does not yet exist. What, then, does the fire signify? The chrism. For the sacrament of the Holy Spirit is the oil of our fire. Notice this when the Acts of the Apostles are read. (Soon the reading of the book is going to begin; today the reader is beginning that book which is called the Acts of the Apostles.) He who wishes to advance has the source of advancement. When you come to church, put aside empty talk; concentrate your attention on the Scriptures. We are your books. Attend, then, and see that the Holy Spirit will come on Pentecost. And thus He will come: He will show Himself in tongues of fire. For He enkindles charity by which we ardently desire God and spurn the world, by which our chaff is consumed and our heart purified as gold. Therefore, the fire, that is, the Holy Spirit, comes after the water; then you become bread, that is, the body of Christ. Hence, in a certain manner, unity is signified.” (St. Augustine, Sermon 227)