In 1 Corinthians, what is sometimes called the "sacrament of sacraments" is described in three ways: The Lord's Supper, Communion, and the Greek word for thanksgiving, Eucharist. From the time Christ said, "Do this is remembrance of Me" until now this sacrament has been at the very center of the Church's life. Everything in the Church points towards the Eucharist, and all things flow from it.
Paul, when writing 1 Corinthians, may have been the first to document what Christ said regarding the Eucharist with these words, "The Lord Jesus on the night when he was betrayed took bread, and when he had given thanks, he broke it, and said, 'This is my body which is for you. Do this in remembrance of me.' In the same way also the cup, after supper, saying, 'This cup is the new covenant in my blood. Do this, as often as you drink it, in remembrance of me'."
The story would be told in all three synopotic Gospels (Matthew, Mark, and Luke), which follows much along the same line as 1st Corinthians. The Eucharist would be left out of John, but he would have much to say about it in Chapter 6 of his Gospel.
When the Apostles were still allowed to attend the Jewish synagogues, the major part that changed in their liturgical cycle was the sacrificing animals. On Sunday, the Lord's day, the animal sacrifice was transformed in the light of Christ to the Eucharistic sacrifice, the sacrifice of Thanksgiving. The Eucharist itself was the focal point of the Christological synagogue that was being formed as evidenced by Justin Martyr, the Didache, and other sources.
Justin Martyr (100-165) in his First Apology, described early church services this way:
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. 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.
In the Didache (around 200 AD), an early document regarding the teaching of the Apostles:
Concerning the Eucharist, give thanks thus: First, concerning the cup: "We give thanks to you, our Father, For the holy vine of David your servant which you have revealed to us through Jesus your servant. To you be glory for ever." And concerning the fragment: "We give thanks to you, our Father, For the life and knowledge, which you have revealed to us through Jesus your servant." But let no one eat or drink of your Eucharist, unless they have been baptized into the name of the Lord. After you have had your fill, give thanks thus: We give thanks to you holy Father for your holy Name which you have made to dwell in our hearts and for the knowledge, faith and immortality which you have revealed to us through Jesus your servant. To you be glory for ever. You Lord almighty have created everything for the sake of your Name; you have given human beings food and drink to partake with enjoyment so that they might give thanks; but to us you have given the grace of spiritual food and drink and of eternal life through Jesus your servant. Above all we give you thanks because you are mighty. To you be glory for ever. Remember Lord your Church, to preserve it from all evil and to make it perfect in your love. And, sanctified, gather it from the four winds into your kingdom which you have prepared for it. Because yours is the power and the glory for ever. ...
It is clear from a cursory look at church history that early Christians has at the very center of their worship the Eucharist, and it was observed every time they gathered on the Lord's day. Biblical scholars from both west and east agree with this, and for the most part, so do Protestants. This was not something just to be added to the worship practice of the early church when they gathered, but the Eucharist was the reason why they gathered.
But what did the early church think the Eucharist was?...
Part 2 - A symbol? | Part 3 - A Sacrifice?