“After we have thus washed the one who has believed and has assented, we lead him to where those who are called brethren are gathered, offering prayers in common and heartily for ourselves and for the one who has been illuminated, and for all others everywhere, so that we may be accounted worthy, now that we have learned the truth, to be found keepers of the commandments, so that we may be saved with an eternal salvation. Having concluded the prayers, we greet one another with a kiss. Then there is brought to the president of the brethren bread and a cup of water and of watered wine; and taking them, he gives praise and glory to the Father of all, through the name of the Son and of the Holy Spirit; and he himself gives thanks at some length in order that these things may be deemed worthy.
When the prayers and the thanksgiving are completed, all the people present call out their assent, saying: “Amen!” Amen in the Hebrew language signifies so be it. After the president has given thanks, and all the people have shouted their assent, those whom we call deacons give to each one present to partake of the Eucharistic bread and wine and water; and to those who are absent they carry away a portion.”
 “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, 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 the blood of that incarnated Jesus.
The Apostles, in the Memoirs which they produced, which are called Gospels, have thus passed on that which was enjoined upon them: that Jesus took bread and, having given thanks, said, “Do this in remembrance of Me; this is My Body.” [cf. Lk 22:19, Mt 26:26, Mk 14:22, 1Cor 11:23-24] And in like manner, taking the cup, and having given thanks, He said, “This is My Blood.” [cf. Lk 22:20, Mt 26:27-28, Mk 14:24, 1Cor 11:25] And He imparted this to them only. The evil demons, however, have passed on its imitation in the mysteries of Mithra. For, as you know or are able to learn, bread and a cup of water together with certain incantations are used in the initiation to the mystic rites.”(Justin Martyr, [First] Apology, 65-66 [A.D. 148 (155)] )
[bracketed Scriptural references provided in The Faith of the Early Fathers by William Jurgens, Volume 1, pp.55, 57]
St. Justin Martyr goes on to explain, in even more detail, the celebration of what Catholics today call “the Mass”.