As to your question about if it has changed. It has changed in some very minor ways, but the structure, and basics have remained completely unchanged sine the Apostles. Take this example from St. Justin Martyr which was written in AD 155 or so. You will note the following order which follows the Mass today.
- The collect, or bringing everyone together.
- Opening prayers / Act of Contrition
- Sign of Peace
- Presentation of Gifts
- Eucharist Prayers
- the Great Amen
- Communion
- Taking Eucharist to the sick and homebound
- Readings from OT and NT
- Homily
*Chapter 65. Administration of the sacraments
But we, after we have thus washed him who has been convinced and has assented to our teaching, bring him to the place where those who are called brethren are
1. assembled, in order that we may
2. offer hearty prayers in common for ourselves and for the baptized [illuminated] person, and for all others in every place, that we may be counted worthy, now that we have learned the truth, by our works also to be found good citizens and keepers of the commandments, so that we may be saved with an everlasting salvation. Having ended the prayers,
3. we salute one another with a kiss. There is then
4. brought to the president of the brethren bread and a cup of wine mixed with water; and he taking them,
5. 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,
6. all the people present express their assent by saying Amen. This word Amen answers in the Hebrew language to γένοιτο [so be it]. 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 **7. partake of the bread and wine mixed with water **over which the thanksgiving was pronounced, and
8. to those who are absent they carry away a portion.
Chapter 66. Of the Eucharist
And this food is called among us Εὐχαριστία [the Eucharist], of which no one is allowed to partake but the man who believes that the things which we teach are true, and who has been washed with the washing that is for the remission of sins, and unto regeneration, and who is so living as Christ has enjoined. For not as common bread and common drink do we receive these; but in like manner as Jesus Christ our Saviour, 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. For the apostles, in the memoirs composed by them, which are called Gospels, have thus delivered unto us what was enjoined upon them; that Jesus took bread, and when He had given thanks, said, “This do in remembrance of Me, Luke 22:19 this is My body;” and that, after the same manner, having taken the cup and given thanks, He said, “This is My blood;” and gave it to them alone. 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.
Chapter 67. Weekly worship of the Christians
And we afterwards continually remind each other of these things. And the wealthy among us help the needy; and we always keep together; and for all things wherewith we are supplied, we bless the Maker of all through His Son Jesus Christ, and through the Holy Ghost. And on the day called Sunday, all who live in cities or in the country gather together to one place,
9. and the memoirs of the apostles or the writings of the prophets are read, as long as time permits; then, when the reader has ceased, the
10. president verbally instructs, and exhorts to the imitation of these good things. Then we all rise together and pray, and, as we before said, when our prayer is ended, bread and wine and water are brought, and the president in like manner offers prayers and thanksgivings, according to his ability, and the people assent, saying Amen; and there is a distribution to each, and a participation of that over which thanks have been given, and to those who are absent a portion is sent by the deacons. And they who are well to do, and willing, give what each thinks fit; and what is collected is deposited with the president, who succours the orphans and widows and those who, through sickness or any other cause, are in want, and those who are in bonds and the strangers sojourning among us, and in a word takes care of all who are in need. But Sunday is the day on which we all hold our common assembly, because it is the first day on which God, having wrought a change in the darkness and matter, made the world; and Jesus Christ our Savior on the same day rose from the dead. For He was crucified on the day before that of Saturn (Saturday); and on the day after that of Saturn, which is the day of the Sun, having appeared to His apostles and disciples, He taught them these things, which we have submitted to you also for your consideration.*
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