In the Book of Acts and in Paul's writing to the church in Corinth, we can get some idea of how the earliest followers of Christ celebrated the Lord's Supper, but neither source provides much detail. Fortunately, there are other written records which help us to flesh out the bare bones of the biblical record. One of them, the Didache, was rediscovered in 1873. The Didache is a manual for church order and Christian living, probably written in Syria around 60 AD. In it, we have the earliest look, outside the New Testament, of the Eucharistic celebration.
The Didache, which recommends praying the Lord's Prayer thrice daily, provides a clear picture of how early Christians would gather on the Lord's Day to "break bread and give thanks," an activity in which only baptized believers were to participate.
The first activity of the day involved confessing their sins and reconciling themselves with their neighbors in preparation for making a pure sacrifice to the Lord. The actual service, which followed orthodox Jewish forms for prayer before and after meals, began with thanksgiving over the cup and the loaf. When offering the cup, the worship leader would give thanks for the "holy vine of David," likely a refererence to the Messianic community (Psalm 80:8), following up with a doxology, "To you be glory forever."
After the doxology, the worship leader would give thanks over the broken loaf, thanking God "for the life and knowledge You have revealed through Jesus, Your Child," concluding with another doxology. That was followed by a community meal which, though not detailed in the Didache, likely was a precursor of the pot luck suppers we see in some modern churches.
How did that simple worship evolve into the carefully choreographed practice of a Roman Catholic Mass? In looking at possible reasons, I think it worth taking another look at the practices of the cult of Mithras, which Christianity replaced as the official state religion of the Roman Empire. Justin Martyr accused the devotees of Mithraism of copying the Christian Eucharistic practice:
Which the wicked devils have imitated in the mysteries of Mithras, commanding the same thing to be done. For, that bread and a cup of water are placed with certain incantations in the mystic rites of one who is being initiated, you either know or can learn.–Justin Martyr, Op. cit,, Chap 66, p. 343
Justin Martyr wrote that the bread and wine of the "Eucharistic sacrifice" were the actual body and blood of Christ. It only took a few years after the Apostles had died off for the infant church to corrupt a simple and non-mystical ceremony of thanksgiving and praise into a pagan festival of human sacrifice, ritual cannibalism and magic.
For not as common bread and common drink do we receive these; but in like manner as Jesus Christ our Savior, 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.–Ibid. p. 343)