This food is called Eucharist among us. No one is allowed to partake of it except those who hold our teachings to be true, have received the washing for the forgiveness of sins and for rebirth, and live according to the instructions of Christ. For we do not receive it as common bread and common drink; but just as Jesus Christ, our Savior, became flesh through God’s Logos and took on flesh and blood for our salvation, so we have been taught that the food consecrated by a prayer of the Logos that comes from him, with thanksgiving, by which our flesh and blood are nourished through transformation, is the flesh and blood of that incarnate Jesus. For the apostles, in the memoirs composed by them, which are called Gospels, have handed down that the following instruction was given to them: Jesus took bread, gave thanks, and said, “Do this in remembrance of me, this is my body,” and likewise he took the cup, gave thanks, and said, “This is my blood,” and he gave it only to them. The evil demons have also imitated this practice in the Mithras mysteries and have given instructions for it. For that bread and a cup of water are set out with the utterance of certain formulas during the initiation of a new disciple, you know or can learn.
Source: Justin Martyr, First Apology, Chapter 66. (155-157 AD)