Essence is properly described as that whereby a thing is what it is...
If we are going to compare the Tridentine Mass to the Novus Ordo Mass we must first properly define what the Mass is in its essence. What exactly takes place during the Mass? Once we can define the essence of the Mass properly, then we can examine the prayers and composition of each Mass and see which best presents this reality to the faithful. This is very important since when we are referring to what the Mass is, it stands to reason that we should do everything possible to present its essence as best we can in the structure of the Mass. Pope Pius XII wrote that the Holy Sacrifice of the Mass is, "the fountain-head of genuine Christian devotion." We will see why this is the case.
The Mass in the West has been traditionally referred to as 'The Most Holy Sacrifice of the Mass.' Unfortunately this term has become somewhat obscured over the past 50 years or so. Most theologians refer to it now just as 'The Mass.' Although it may be acceptable not to have to write out the name in long form throughout an entire article or writing, it is very unfortunate that this full explanatory term is rarely used at all today. Essence is simply the definition of a thing, what it is. One of the most extreme travesties caused by many modern theologians is that they no longer define anything. They often use terms loosely not wanting to be pinned down by dogmatic language, and as a result their theology mimics an amoebic form rather than a structured form. When we explain the Holy Sacrifice of the Mass in concrete terms it is only then that the intellect can properly comprehend what is truly is.
Saint Thomas Aquinas gives an explanation as to the essence of the Mass in the Summa Theologica, "...the celebration of this sacrament is an image representing Christ's Passion, which is His true sacrifice. Accordingly the celebration of this sacrament is called Christ's sacrifice...As the celebration of this sacrament is an image representing Christ's Passion, so the altar is representative of the cross itself, upon which Christ was sacrificed in His proper species." There are many who now say that the Mass is a meal, in which Christ offers Himself as He did at the Last Supper. Yet they fail to realize that the Last Supper itself is His Sacrifice in a real and proper sense. In other words the Last Supper is in reality Christ's very sacrifice on the cross, they cannot be separated. The Body which we receive is truly that Body which Jesus Christ offered up on the cross, as well as being present at The Last Supper. To refer to the Holy sacrifice of the Mass as a meal as its essence would be incorrect. It is only a meal in the context of the whole sacrifice, which is why when modernists refer to the Mass as a meal in relation to the Last Supper and the Mass, they are completely missing the point.
All of the apostolic Churches understand The Last Supper in a sacrificial manner including the Eastern Orthodox. For example, in Heiromonk Gregorios' commentary on the Liturgy of St. John Chrysostom he writes, "At the Last Supper, Christ celebrates sacramentally His sacrifice on the cross." As Cardinal Journet explained, Saint Cyprian of Carthage illustrated this point in his Letter LXIII to Cecil where he clearly spoke of the Mass as being the very same sacrifice as that of the Last Supper, and that of Christ's passion. The meal or "Supper" aspect of eating Christ's flesh cannot be separated from His sacrifice, and henceforth the sacrifice is the essence of the Mass. It is what the Mass is. To call the Mass a meal would be like calling a Holy Icon of the Virgin Mary a wooden board. The Holy Icon may be composed of a wooden board, but it is in its essence, a Holy Icon. Likewise the Holy Sacrifice of the Mass may have elements of a meal by our eating of His Body and Blood, but it is in its essence a sacrifice, or THE sacrifice.
We must realize that the sacrifice that is taking place on the altar is the exact same sacrifice that happened in Jerusalem 2000 years ago, given under the mysterious veil of the Sacrament. Father John Hardon once wrote, "Trent tells us that the sacrifice of the Mass is not only a liturgical ceremony, or merely a celebration or merely a remembrance of the sacrifice on Calvary. No, the Mass is a sacrifice. The Mass is the sacrifice, which St. Paul tells us wiped out all the other sacrifices that had been offered until the coming of Christ. Christ’s death on the cross originally merited the graces to redeem the world, but Christ now actually confers those graces." Can this be any clearer? Christ being high priest and victim, presents Himself in the same manner of sacrifice, that of the cross, wherever the Holy Sacrifice of the Mass is taking place here now on earth. It must be understood that not only is Christ present in His full substance, but His very sacrifice and passion is also truly present. So we must take care not to equate the essence of the Mass to a meal, as if Christ is only giving Himself to be eaten or consumed.
If we are not careful we may ignorantly fall into the condemnation of Trent, "If anyone says that in the Mass a true and real sacrifice is not offered to God, or that the act of offering is nothing else than Christ being given to us to eat, let him be anathema." Notice this condemnation is not related to one denying the Real Presence of Christ in the Mass, but the manner which He is present. If one were to believe that Christ was substantially present in the Mass in His Body, Blood, Soul and Divinity, and yet deny the essential manner in which He is present, which is that of sacrifice, one would be a heretic. Once we come to understand this reality it should give us pause as to how we must view the prayers, rubrics and setting of the Mass that surrounds the consecration, and to how they relate to the manner in which Christ is present in the consecration.