An evangelical worship service these days usually goes something like the following: There's about 30 min of praise music led by a band, and then a 30-45min sermon. There might be short, extemporaneous prayers said by various leaders at the front at different points. The Lord's Supper is generally celebrated somewhere between once a month and a few times a year, and the bread and the wine is usually held to be merely symbolic.
The Catholic Mass has this basic outline: The Scriptures are read aloud, there's a homily, the congregation offers prayers together as a congregation, the Eucharistic prayers are offered by the priest to which the congregation responds with the great 'Amen', and finally the Eucharist is distributed to all baptized Catholics in good standing in the Church. The Eucharist, or Lord's Supper, is always celebrated, and it is held that the bread and the wine actually become the true Body and Blood of Jesus.
But how did the early Christians worship?
We actually know. St Justin the Martyr was a 2nd century philosopher who converted to Christianity. Christianity was outlawed in the Roman Empire, but Justin believed that this was the case largely due to misunderstandings of Christianity held by the Roman officials. So he wrote a letter around A.D. 150 to the Emperor which we refer to today as The First Apology. In it he explains the Christian faith of his day. At the end of this letter, he describes when and how Christians worshiped in his day:
[O]n the day called Sunday, all who live in cities or in the country gather together to one place, 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 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. (67)
Sound familiar? It's because its the same thing that Catholics do today, 1900 years later.
He also describes more clearly what the early Christians believed about the nature 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, 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. (66)
(Sidenote: He clearly believes that baptism does something, that it's not just a symbol. When a person is baptized, there is "remission of sins" and "regeneration".)
He calls it the Eucharist. He specifically says that the bread and wine are received "not as common bread and common drink", but that this food "is the flesh and blood of that Jesus". He then goes on to prove this by quoting Jesus when he said "This is my body" and "This is my blood" - words that evangelicals dismiss, without Scriptural warrant, as merely symbolic.
The Mass, the Catholic belief in the real presence - these were not late medieval corruptions. They have been there from the very beginning. Evangelicals are the ones who have done something new.
Evangelical worship would have been foreign to the early Christians. The Cathoic Mass, in its fundamental form, is how the early Church worshiped, as passed down from the Apostles.