While there exist many different liturgical rites within the Catholic Church, each with their own unique traditions, the basic structure of the Mass across all these rites is the same as it it was in the 2nd century. When compared side-by-side, surprisingly little has changed since the early Christians celebrated the Eucharist in the catacombs and homes of ancient Rome.
The Introductory Rites
“The rites that precede the Liturgy of the Word … have the character of a beginning, an introduction, and a preparation. Their purpose is to ensure that the faithful, who come together as one, establish communion and dispose themselves properly to listen to the Word of God and to celebrate the Eucharist worthily.” (GIRM, 46)
And on the day called Sunday, all who live in cities or in the country gather together to one place. (The First Apology, 67)
The Liturgy of the Word
“In the readings, the table of God’s Word is spread before the faithful, and the treasures of the Bible are opened to them. Hence, it is preferable that the arrangement of the biblical readings be maintained, for by them the unity of both Testaments and of salvation history is brought out … The Homily is part of the Liturgy and is highly recommended, for it is necessary for the nurturing of the Christian life. It should be an explanation of some aspect of the readings from Sacred Scripture or of another text from the Ordinary or the Proper of the Mass of the day and should take into account both the mystery being celebrated and the particular needs of the listeners.” (GIRM, 57, 63)
[T]he memoirs of the apostles or the writings of the prophets are read, as long as time permits; then, when the reader has ceased, the president verbally instructs, and exhorts to the imitation of these good things. (The First Apology, 67)