This was written by Saint Justin, martyr around the year 155 AD proving the Catholic Mass we celebrate today was celebrated at the earliest times of Christianity and from the beginning and it came from our Lord and Savior Jesus The Christ starting from on the night he was betrayed and what we today call The Last Supper and was handed down to us by The Apostles, see this link- http://www.thedidache.com/
Justin, martyr was writing this to and for the authorities and Roman governmental officials who were questioning and wanted to know what The Christians were doing and why and it’s the reason we still do it today.
From the first apology in defense of the Christians by Saint Justin, martyr (Cap. 66-67: PB 6, 427-431) The celebration of the eucharist
No one may share the eucharist with us unless he believes that what we teach is true, unless he is washed in the regenerating waters of baptism for the remission of his sins, and unless he lives in accordance with the principles given us by Christ.
We do not consume the eucharistic bread and wine as if it were ordinary food and drink, for we have been taught that as Jesus Christ our Savior became a man of flesh and blood by the power of the Word of God, so also the food that our flesh and blood assimilates for its nourishment becomes the flesh and blood of the incarnate Jesus by the power of his own words contained in the prayer of thanksgiving.
The apostles, in their recollections, which are called gospels, handed down to us what Jesus commanded them to do. They tell us that he took bread, gave thanks and said: Do this in memory of me. This is my body. In the same way he took the cup, he gave thanks and said: This is my blood. The Lord gave this command to them alone. Ever since then we have constantly reminded one another of these things. The rich among us help the poor and we are always united. For all that we receive we praise the Creator of the universe through his Son Jesus Christ and through the Holy Spirit.
On Sunday we have a common assembly of all our members, whether they live in the city or the outlying districts. The recollections of the apostles or the writings of the prophets are read, as long as there is time. When the reader has finished, the president of the assembly speaks to us; he urges everyone to imitate the examples of virtue we have heard in the readings. Then we all stand up together and pray.
On the conclusion of our prayer, bread and wine and water are brought forward. The president offers prayers and gives thanks to the best of his ability, and the people give assent by saying, “Amen.” The eucharist is distributed, everyone present communicates, and the deacons take it to those who are absent.
The wealthy, if they wish, may make a contribution, and they themselves decide the amount. The collection is placed in the custody of the president, who uses it to help the orphans and widows and all who for any reason are in distress, whether because they are sick, in prison, or away from home. In a word, he takes care of all who are in need.
We hold our common assembly on Sunday because it is the first day of the week, the day on which God put darkness and chaos to flight and created the world, and because on that same day our savior Jesus Christ rose from the dead. For he was crucified on Friday and on Sunday he appeared to his apostles and disciples and taught them the things that we have passed on for your consideration.