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Early Christian sources

The Didache (Greek: Διδαχή, "teaching") is an Early Church treatise that includes instructions for baptism and the Eucharist. Most scholars date it to the late 1st century, and distinguish in it two separate Eucharistic traditions, the earlier tradition in chapter 10 and the later one preceding it in chapter 9. The Eucharist is mentioned again in chapter 14.

Ignatius of Antioch (born c. 35 or 50, died between 98 and 117), one of the Apostolic Fathers, mentions the Eucharist as "the flesh of our Saviour Jesus Christ":

They abstain from the Eucharist and from prayer, because they confess not the Eucharist to be the flesh of our Saviour Jesus Christ, which suffered for our sins, and which the Father, of His goodness, raised up again. Let that be deemed a proper Eucharist, which is either by the bishop, or by one to whom he has entrusted it.

Take heed, then, to have but one Eucharist. For there is one flesh of our Lord Jesus Christ, and one cup to the unity of His blood; one altar; as there is one bishop, along with the presbytery and deacons, my fellow-servants: that so, whatsoever you do, you may do it according to God.

Justin Martyr (born c. 100, died c. 165) mentions in this regard:

And this food is called among us Εὐχαριστία, of which no one is allowed to partake but the man who believes that the things which we teach are true, and who has been washed with the washing that is for the remission of sins, and unto regeneration, and who is so living as Christ has enjoined. For not as common bread and common drink do we receive these; but in like manner as Jesus Christ our Saviour, having been made flesh by the Word of God, had both flesh and blood for our salvation, so likewise have we been taught that the food which is blessed by the prayer of His word, and from which our blood and flesh by transmutation are nourished, is the flesh and blood of that Jesus who was made flesh.

Paschasius Radbertus (785–865) was a Carolingian theologian, and the abbot of Corbie, whose best-known and influential work is an exposition on the nature of the Eucharist written around 831, entitled De Corpore et Sanguine Domini. In it, Paschasius agrees with St Ambrose in affirming that the Eucharist contains the true, historical body of Jesus Christ. According to Paschasius, God is truth itself, and therefore, his words and actions must be true. Christ's proclamation at the Last Supper that the bread and wine were his body and blood must be taken literally, since God is truth. He thus believes that the transubstantiation of the bread and wine offered in the Eucharist really occurs. Only if the Eucharist is the actual body and blood of Christ can a Christian know it is salvific.