We call this food Eucharist; and no one else is permitted to partake of it, except one who believes our teaching to be true and who has been washed in the washing which is for the remission of sins and for regeneration [being born again in Baptism], and is thereby living as Christ has enjoined.

For not as common bread nor common drink do we receive these; but since Jesus Christ our Savior was made incarnate by the word of God and had both flesh and blood for our salvation, so too, as we have been taught, the food which has been made into the Eucharist by the Eucharistic prayer set down by Him, and by the change of which our blood and flesh is nourished, is both the flesh and blood of that incarnate Jesus.  ~ Saint Justin Martyr, A.D. 150 (First Apology 66)