If you some of the link, Timmy, you would see one of the explanation.
I shall extract some of that information.
Christ’s bloody sacrifice on Calvary took place once, and it will never be repeated. To repeat his sacrifice would be to imply that the original offering was defective or insufficient, like the animal sacrifices of the Old Testament that could never take away sins. Jesus’ offering was perfect, efficacious, and eternal.
The Mass is a participation in this one heavenly offering. The risen Christ becomes present on the altar and offers himself to God as a living sacrifice. Like the Mass, Christ words at the Last Supper are words of sacrifice, “This is my body . . . this is my blood . . . given up for you.” So, the Mass is not repeating the murder of Jesus, but is taking part in what never ends: the offering of Christ to the Father for our sake (Heb 7:25, 9:24). After all, if Calvary didn’t get the job done, then the Mass won’t help. It is precisely because the death of Christ was sufficient that the Mass is celebrated. It does not add to or take away from the work of Christ—it is the work of Christ.
Question 4 of the article mention in this link states;
Before looking at the ancient beliefs of Christians, consider the modern belief. This may come as a shock to Evangelicals, but seventy-five percent of Christians believe the Mass to be a sacrifice. When you add up the Catholics, Orthodox, Abysinnians, Coptics, Assyrians, Armenians, et cetera, it is evident that Protestants are in the minority with their interpretation. Still, it’s fallacious to argue that because a majority of people believes something it must be true, so let’s examine the faith of the first Christians.
The Didache written in 70 AD states,
The Didache refers to the Eucharist as a thusia, the Greek term for sacrifice: "Assemble on the Lord’s day, and break bread and offer the Eucharist; but first make confession of your faults, so that your sacrifice may be a pure one. Anyone who has a difference with his fellow is not to take part with you until they have been reconciled, so as to avoid any profanation of your sacrifice [Matt. 5:23–24]. For this is the offering of which the Lord has said, ‘Everywhere and always bring me a sacrifice that is undefiled, for I am a great king, says the Lord, and my name is the wonder of nations’ [Mal. 1:11, 14]" (Didache 14 [A.D. 70]).
Justin Martyr added these thoughts with regard to the prophecy of Malachi: “God speaks by the mouth of Malachi, one of the twelve [minor prophets], as I said before, about the sacrifices at that time presented by you: ‘I have no pleasure in you, says the Lord, and I will not accept your sacrifices at your hands; for from the rising of the sun to the going down of the same, my name has been glorified among the Gentiles, and in every place incense is offered to my name, and a pure offering, for my name is great among the Gentiles’ [Mal. 1:10–11]. He then speaks of those Gentiles, namely us [Christians] who in every place offer sacrifices to him, that is, the bread of the Eucharist and also the cup of the Eucharist” (Dialogue with Trypho the Jew 41 [A.D. 155]).
St. Ireneaus, the disciple of St. Polycarp, disciple of St. John, the Apostle wrote this.** “He took from among creation that which is bread, and gave thanks, saying, ‘This is my body.’ The cup likewise, which is from among the creation to which we belong, he confessed to be his blood. He taught the new sacrifice of the new covenant, of which Malachi, one of the twelve [minor] prophets, had signified beforehand. . . .He makes it plain that the former people will cease to make offerings to God; but that in every place sacrifice will be offered to him, and indeed, a pure one, for his name is glorified among the Gentiles”**
I don’t know how you would discredit the ECF, but in the Gospels, the Apostles, that the first generation of Christian would remain faith to everything which the Apostles taught, whether they were oral or written. St. Paul himself made this very clear. Polycarp would be the first generation, since he was taught by St. John, Polycarp in turn taught, Ireneaus (2nd generation).