Didache (1st Century) 9:1-5:
Now about the Eucharist: This is how to give thanks: First in connection with the cup: "We thank you, our Father, for the holy vine of David, your child, which you have revealed through Jesus, your child. To you be glory forever."
Then in connection with the piece [broken off the loaf]: "We thank you, our Father, for the life and knowledge which you have revealed through Jesus, your child. To you be glory forever.
"As this piece [of bread] was scattered over the hills and then was brought together and made one, so let your Church be brought together from the ends of the earth into your Kingdom. For yours is the glory and the power through Jesus Christ forever."
You must not let anyone eat or drink of your Eucharist except those baptized in the Lord's name. For in reference to this the Lord said, "Do not give what is sacred to dogs."
Ignatius’ Letter to the Smyrneans (Late 1st Century or Early 2nd Century) Chapter 7:
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. Those, therefore, who speak against this gift of God, incur death in the midst of their disputes. But it were better for them to treat it with respect, that they also might rise again. It is fitting, therefore, that ye should keep aloof from such persons, and not to speak of them either in private or in public, but to give heed to the prophets, and above all, to the Gospel, in which the passion[of Christ] has been revealed to us, and the resurrection has been fully proved. But avoid all divisions, as the beginning of evils.
Justin Martyr’s Dialog with Trypho (Mid-2nd Century) Chapter 31:
"And the offering of fine flour, sirs," I said, "which was prescribed to be presented on behalf of those purified from leprosy, was a type of the bread of the Eucharist, the celebration of which our Lord Jesus Christ prescribed, in remembrance of the suffering which He endured on behalf of those who are purified in soul from all iniquity, in order that we may at the same time thank God for having created the world, with all things therein, for the sake of man, and for delivering us from the evil in which we were, and for utterly overthrowing principalities and powers by Him who suffered according to His will."
Justin Martyr’s Dialog with Trypho (Mid-2nd Century) Chapter 117:
…the Eucharist of the bread and the cup…in the remembrance effected by their solid and liquid food, whereby the suffering of the Son of God which He endured is brought to mind.
Justin Martyr’s First Apology (Mid-2nd Century) Chapters 65-66:
But we, after we have thus washed him who has been convinced and has assented to our teaching, bring him to the place where those who are called brethren are assembled, in order that we may offer hearty prayers in common for ourselves and for the baptized [illuminated] person, and for all others in every place, that we may be counted worthy, now that we have learned the truth, by our works also to be found good citizens and keepers of the commandments, so that we may be saved with an everlasting salvation. Having ended the prayers, we salute one another with a kiss. There is then brought to the president of the brethren bread and a cup of wine mixed with water; and he taking them, gives praise and glory to the Father of the universe, through the name of the Son and of the Holy Ghost, and offers thanks at considerable length for our being counted worthy to receive these things at His hands. And when he has concluded the prayers and thanksgivings, all the people present express their assent by saying Amen. This word Amen answers in the Hebrew language to genoito [so be it]. And when the president has given thanks, and all the people have expressed their assent, those who are called by us deacons give to each of those present to partake of the bread and wine mixed with water over which the thanksgiving was pronounced, and to those who are absent they carry away a portion.
And this food is called among us Eukaristia [the Eucharist], 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. For the apostles, in the memoirs composed by them, which are called Gospels, have thus delivered unto us what was enjoined upon them; that Jesus took bread, and when He had given thanks, said, "This do ye in remembrance of Me, this is My body;" and that, after the same manner, having taken the cup and given thanks, He said, "This is My blood;" and gave it to them alone.
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Find more of what the early Christians thought on my Christian History page!