According to Trent, and not quite according to the Catechismus Romanus the sacrifice of the Mass is not a repetition of the sacrifice on the cross, but its remembrance, its being made present, and its application. Nevertheless, the sacrifice of the Mass is said to be in fact a propitiatory sacrifice. And since in this divine Sacrifice which is performed in the Mass, that same Christ is contained in a bloodless sacrifice who on the altar of the cross once offered himself with the shedding of his blood: the holy Synod teaches that this sacrifice is truly propitiatory. … God, propitiated by the oblation of this sacrifice …This however does not succeed in setting aside what is most repugnant in the notion of the identity of the sacrifice of the Mass with the sacrifice on the cross. That is the assertion that we human beings sacrifice the body and blood of Christ.
Luther had asked the question, to get the Papists out in the open on the sacrifice of the Mass, "Why do you cling so tenaciously to the Mass?"
Philip Melanchthon is quite clear about this in his Apology, Article 15 [On Human Traditions]. It is because the papal party has substituted a human worship of God that they prefer to the one commanded by God. That is, they do not like the promises given in Christ, and prefer others.
Philip was, oddly enough, shocked that his distinction in AC 15 [Concerning Church Rites] between human traditions and God's own institution of true worship in pure preaching and right administration of sacraments would ever be questioned. He writes that, "we never dreamed that they [the papal party] would actually condemn the proposition that we do not merit the forgiveness of sins or grace by observing human traditions" (Kolb-Wengert 223.3). He then goes on to note that the opponents "appoint another mediator" than Christ. (223.5) They add to Christ's last will and testament (223.12). Then they "deprive Christ of his honor," acting as if Christ is absent, not present (225.18). Then what happens? Well, the dreamed-up spiritual life becomes something preferred by the religious, becoming "far preferable to the works that God commands," (227.25), which is the actual vocation (227.25).
But back to Luther's question, "Why do you cling…?" Because they have set up an illusion, which they prefer, to the true worship, a righteousness that comes through divine law, not promises. This can be seen in three key Roman Catholic teachings about sacraments:
The Sacramental System understood grace and the means of grace under the general category of eternal law. According to this thinking, the sacraments were "new laws," that fulfilled and replaced the old laws. That is, circumcision was replaced and completed by baptism, the Passover superceded by the Lord's Supper, as Thomas Aquinas writes in Summa Gentiles. Priests were understood as "for" priests, offering a sacrifice that completed and replaced that of the Levites. And each sacrament was instituted by God for a stage of development on the way back to God from vice to virtue. Baptism to get rid of original sin, Confirmation for adolescents becoming responsible for actual sins, the Lord's Supper and Penance for continual growth, the Orders or Marriage for entering upon adult life, and Final Unction for the preparation for new life.
Yet, perhaps even more importantly, understanding sacraments under the law meant creating an office of the papacy that played a necessary role in salvation. Even if it was not initially demanded by Scripture or the Apostolic Church, the argument goes, the office can become irreversable and necessary as the Spirit works through the tradition of the Church to secure the Gospel message, as Karl Rahner noted. When you begin salvation with law, and end it with law, then one of the main necessities is to have some way of extending and elaborating law over time. The Pharisees did it with an oral tradition, and so it survived among all types of Jews, because they were able to extend law indefinitely.
For Rome, the Pope was understood as being the means by which God's law could be extended, enlarged, and applied especially to Christians in a new place and time. The pope's ability to make law that is binding becomes the source of canon law, and the means by which he is understood to do this is by the perceived necessity of having a final and ultimate authority to interpret Scripture when so many interpretations compete for the hearts of people. Scripture is then taken as a book of mysteries, which has applications in it for future ages planted by God, but not yet understood by its authors who lived in its history. In this way the church itself was understood to be a kind of ongoing incarnation of Christ. A means by which God's law is extended indefinitely into the future. A sacrament itself. After all, when Christ is no longer around to tell you what to do each time some new moral conundrum arises, to whom can you turn? Such teaching authority is absolutely crucial for any understanding of salvation that comes through the law.
Right at this point the Lutherans unearthed another reason why this mass is clung to so tightly. Christ had come to be understood as in some important way absent, since he is visibly and physically apparently gone. Yet, Het has left us with a new and improved version of the law to fulfill. What are we to do?
We all know that we cannot fulfill the law on our own without Christ and grace, but how will we know His will in any time or place as new situations constantly arise?
In steady steps the notion of mediation between Christ in heaven and his earthly presence begins to solidify around the office of the Papacy as the Vicar of Christ. The Pope becomes the stand-in for Christ, whose power is extended down to the very parish priest through ordination.
The key means of mediation then is to offer up an unbloody sacrifice, as it came to be called, an acceptable request that Christ re-present Himself despite the unholiness of this world, its people and this very Church. A sacrifice of prayer is to be offered that pleases God, and is met by God with approval, and a re-presenting of Christ, who thus comes into the elements or publishes his eternal will into the Church. That is, the head of the Church, Christ, returns to his body, the Church, and in this unity makes the fulfillment of the law as righteousness a reality again.
Why cling to the mass as an unbloody sacrifice? In order to get an absent Christ to re-present himself as a means of grace and not only judgment.
Notice what happens to the understanding of "Word," in this sense of using the Mass. The "Word" in the Sacrament then was either turned into mere prayer to God, with its implied exhortation to the gathered people (in the line of via moderna, who understood that God's will before all time needed to be agreed with), or it was applied to the bread and wine by means of manufacturing a holy place in this unholy world that could harbor Christ really present again (Thomas Aquinas' direction in both baptism and Lord's Supper).
Too often the modern dialogs on the "sacrifice" of the Mass have dissembled into ambiguity, noting that something new is happening for Lutherans, so much so that for some Lutheran and Roman Catholics, they seem to be willing to move away from addressing the harsh edges of "sacrifice" in the modern Mass. Then we can say both Lutherans and Roman Catholics understand there is some type of sacrifice going on: at least a sacrifice of praise. But this doesn't get to the real matter. Roman Catholics cling so tightly to the sacrifice because it is a product of the divine right of the papacy de iure divino (SA III. 4, p.307.1). And that is clung to so tightly because when no distinction of law and Gospel is made, there is no other means by which Christ's human and divine nature can stay together, no other graceful way the infinite divine can somehow affect matters on the finite earth, unless we have a mediator who allows the movement between the two of these.
When there is no real new kingdom of Christ into which we are swept by his death and resurrection, then there is only the attempt to improve this world by way of the Church. In other words, at the heart of this thinking about the Mass as a sacrifice there is a fear that Christ is absent, and must be made present, but only can be so by one who stands in an office that is more than human, more than mere human tradition, more than earthly. Without this, like loosing a kite from a broken string, we would loose our head the Christ. This means a significant disruption is woven into the fabric of our understanding of the Church, Pope, and Scripture, to say nothing of the Sacraments, when the Lutherans come into the picture pointing out that Christ himself is the end of the law, and that what lies beyond law is a new kingdom, a new world of Gospel without law. And further yet, Christ is present for us in His very word of promise which puts to death the old sinner and raises the new saint, utterly disturbing the ontological order of having an infinite, almighty God standing over finite an unholy people.
If Christ himself is present, and even as a very specific promise for us in the Sacraments, then the law would be over where He has given His promise. In Christ a new time, a new kingdom, a new life is being created, out of nothing. But that would mean we need no vicar of Christ in the sense of a stand-in, who develops new laws to apply to our worship, and our moral life, etc., as far as concerns our salvation.
Further yet, we would need no protected, authoritative interpreters of Scripture. All we would need is the external preaching office and the hearing of that preaching, which offers Christ as a promise. But then all the added prayers in the Mass are not divine law, or necessary matters for salvation, they are at most human traditions, and so is the papacy itself. At the best we could say the Mass prayers were "aven," that is, "wasted effort." But as Luther noted, monasteries, and the Mass, and the Papacy are all human inventions that are not commanded, not necessary, not useful, and all the "while causing dangerous and futile efforts besides." (306.2)
But who then will make new laws for us regarding salvation? No one. We would then be living beyond the law, in freedom, and there is only that one voice in the conscience that gives it proper location here on earth, "I am pleased with you." That is why Luther notes, "if the Mass fails, the papacy falls." (303.10) "Before they would allow that to happen, they would kill us all, if they could do it." Why? Well, the answer would be self defense. But that is always the last defense of the sinner again the full, all sufficient grace of God in Christ given as a gift for faith alone.