Even though many Catholics do not understand the Mass, it is good for a Christian to know as much about it as possible. From What and Why of Catholicism, Imprimatur Cardinal Spellman, "The sacrifice of the Mass forms a pivot upon which all else turns. If it is what Catholics believe it is, it is the greatest manifestation of the love of God for man and the most magnificent testimonial to the validity of Catholicism; but if it be false, it is the worst farce and blasphemy ever perpetrated upon God or man, and the Catholic faith collapses into nothingness."
From The Catholic Catechism by John Hardon, S.J., "The sacrifice on the altar is no mere commemoration of Calvary, but a true and proper act of sacrifice whereby Christ, the high priest, by an unbloody immolation offers himself a most acceptable victim to the eternal Father, as he did on the cross." (page 466)
Catholics admit that the Mass has developed over the centuries. Priest Karl Adam, in The Spirit of Catholicism, "We Catholics acknowledge readily without shame, nay, with pride, that Catholicism cannot be identified simply and wholly with primitive Christianity."
One of the major points of development was the doctrine of Transubstantiation. This was explained by the greatest Roman Catholic theologian, St. Thomas Aquinas, using the philosophy of Aristotle. Aristotle taught that everything has two properties which he called Essence and Appearance. Aquinas renamed these Substance and Accidents.
Transubstantiation can be explained by breaking up the word. "Substance" is the root, and "trans" means to go over (transcontinental, one side of the continent to the other). "ation" is the part of the word that speaks of action. Transubstantiation is the action of something going from one substance to another.
They believe the wafer BECOMES Jesus Christ. The substance has changed from a wafer to Christ, while the appearance, or accidents, remain the same. They say this is proved by proving, as Aristotle did, that accidents can change while substance remains the same. Fifty years from now your appearance will have changed, but you will still be substantially the same person. Water can be frozen or boiled; in either case, the substance remains the same but the accidents change.
Thomas Aquinas reasoned that if accidents can change while substance remains the same, the reverse can also be true - accidents can remain the same while substance changes. Although the former can be proved, the latter cannot be, but Aquinas said that since God can do anything, and it was fitting that He perform transubstantiation, He did it!
After Vatican 2, Paul VI introduced the New Mass - communal aspect stressed, still the same. There are still the four vital parts of the Mass intact - offertory, consecration, elevation, and the priest consuming the wafer. They often sing Protestant hymns, like Amazing Grace
The Mass has been called a repetition of Calvary. But repetitions are in order only when the initial act did not complete the task (Illus.: if Im knock at your door and you don't come,. I will repeat my knock. But when you come to the door I don't keep knocking.)
Mass has been called a continuation of Calvary, but if a serial story in a magazine says "To be continued", it doesn't say "The End" on the same page. Jesus Christ said "It is finished"; the priest says, "To be continued."
Mass has been called a re-enactment of Calvary, but a school reenacting the signing of the Declaration of Independence does not declare war on England.
Mass has been a memorial, but a war memorial does not win the war.
Mass has been called a demonstration of Calvary, as if God rolled back the years to demonstrate what Jesus did on Calvary. If He did, the Mass would have equal authority and power as the initial sacrifice, and yet no Catholic receives infinite salvation by going to Mass.
Mass is the antithesis of Calvary - the priest disproves the Mass by repeating it. An Infinite Savior demolishes the Mass.
The Catholic Church, which is supposed to never change, put out an authoritative (but not infallible) book in 1913. It was written by Priest Joseph Baierl and published by The Seminary Press complete with Nihil Obstat and Imprimatur.