The Breviary Mattins lections in this time of the Corpus Christi octave continue to be a wonderful harvest from the field of patristic teaching on the Eucharist. Today has this passage from St Ambrose of Milan (c.340-397) writing in De sacramentis:
Bk. iv. ch. 4.
Who invented the Sacraments but the Lord Jesus? The Sacraments came down from heaven, for all counsel is from heaven. Nevertheless, it was a great and wonderful work of God when He rained down manna upon His people, and the people laboured not, and yet were fed. Perchance, thou sayest Here, it is my bread which is used. But that bread is bread only till the Sacramental words are spoken at the Consecration, instead of bread, there cometh to be the Body of Christ. This therefore let us establish. How cometh it that that which was bread becometh the Body of Christ Through the Consecration. And in what words and in Whose language doth the Consecration take place In those of the Lord Jesus. All the other things which are said (in the Liturgy), the ascription of praise to God (in the Preface), the prayer for the people, for kings, and for others which formeth the first part [of the Canon, these are put in the mouth of the Priest.] But when that point is reached when this worshipful Sacrament is to be consecrated, then the Priest useth no more his own words, but Christ's.
It is the word of Christ, therefore, Which doth the needful work in this Sacrament. And what is the word of Christ? It is the word of Him at Whose bidding all things were made. The Lord commanded, and the heavens were created the Lord commanded, and the earth was formed the Lord commanded, and the seas were made the Lord commanded, and all creatures sprang into being. Thou seest, then, how mightily working a word is the word of Christ. If then the word of Christ hath such power that it can make that to be which hath never been, wherein doth it appear greater that it maketh one thing to be changed into Another There was once no heaven there was once no sea there was once no earth. But hear him who saith: "He spake, and it was done He commanded, and it stood fast." Ps. xxxii. 9. If, then, I am to answer thee, I tell thee, that before the Consecration it is not the Body of Christ, but after the Consecration it is the Body of Christ, for Himself "hath spoken, and it is done He hath commanded, and it standeth fast."
And now I come back to my text. It is indeed a great and worshipful fact that manna was rained down upon the Jews but, think thou, which was the more great and worshipful, the manna from heaven or the Body of Christ ”the Body of that Same Christ by Whom the heavens were made And, again the fathers "did eat manna, and are dead he that eateth of this Bread," John vi. 58, It is unto him "the remission of sins," Matth. xxvi. 28, and "he shall never die." John xi. 26.Therefore it is not idly that, when thou art receiving, thou sayest "Amen" testifying in thine heart that That Which thou art taking is the Body of Christ. The Priest saith unto thee: "The Body of Christ" ”and thou answerest: "Amen" that is to say, "It is true." What then thy tongue confesseth, let thine heart hold to.
Quite apart from its forceful eloquence this is, by reason its author and therefore its antiquity a splendid rebuttal of those, such as some sixteenth century religious “reformers” who argue Catholic belief in Transubstantiation dates only from Lateran IV in 1215.