The Catholic Mass
The Mass of the Catholic Church is the centre of her members lives. In the Mass the people of God first hear the Word of God, and then each member becomes united with Christ. The Word of God is read from the Bible. After this is done, The Lord's Supper is celebrated. The Lord's Supper is a reenactment of the Last Supper of Jesus as he commanded his Church to do. This is related to us in three of the four Gospels: Matthew 26:26-30, Mark 14:22-26, and Luke 22:14-20, which I will now recount to you:
And when the hour came, he sat at table, and the apostles with him. And he said to them, "I have earnestly desired to eat this Passover with you before I suffer; for I tell you I shall not eat it until it is fulfilled in the kingdom of God." And he took a cup, and when he had given thanks he said, "Take this, and divide it among yourselves; for I tell you that from now on I shall not drink of the fruit of the vine until the kingdom of God comes." And he took bread, and when he had given thanks he broke it and gave it to them saying, "This is my body which is given for you. Do this in remembrance of me." And likewise the cup after supper, saying, "This cup which is poured out for you is the new covenant in my blood."
When the Church celebrates the Lord's Supper, she fulfills Christ's command to "do this in remembrance of me." When the blessing is said over the bread and wine, Christ's words are fulfilled, "This is My Body," and "This is My Blood." Christ performs a miracle and the bread and wine, although they retain the physical characteristics of bread and wine, are no longer bread and wine, but the Holy Body and Blood of Christ. When Christ's members eat his Holy Body, and drink his Holy Blood, they become united with Christ. This is the most beautiful communion we have with Christ, until we are with him in heaven, where we will be in perfect communion with Christ forever.
In the Catholic Mass, Christ is crucified all over again
Christ was crucified once and for all on Golgotha. This was the universal, absolute Sacrifice for the salvation of the world. Christ commanded that His bloody sacrifice on the Cross should be daily renewed by an unbloody sacrifice of His Body and Blood in the Mass under the simple elements of bread and wine. In the Mass, Christ eats of the Passover again, and drinks of the fruit of the vine because by Him "the kingdom of God comes." This alone is the origin and nature of the Mass.
Christ is not crucified again in the Mass, but his one crucifixion is renewed every day in the Catholic Mass. The sacrifice in the Mass is the same sacrifice as on Golgotha, just as it is the same sacrifice in the Last Supper before the crucifixion. In all three places, it is the same sacrifice; the same crucifixion. God is acting outside of time here, making the one sacrifice present at different locations in time. This is emphasized in Christ's words at the Last Supper: "which is given for you," and "which is poured out;" not "which will be given for you," and "which will be poured out." The Last Supper was not a prophecy of the crucifixion, but is the crucifixion; just as the Mass is the crucifixion. Christ is not re-crucified during the Mass; however, Christ's one crucifixion is present in the Mass.
The bread and wine in the Catholic Mass are a sacrifice like the old covenant Jewish sacrifices
The sacrifices of the Old Testament, by their figurative forms and prophetic significance, point to the sacrifice of the Cross as their eventual fulfillment. The Sacrifice of the Mass is an anticipated commemoration of the sacrifice of the Cross. We do not offer the bread and wine on the altar as a sacrifice for our sins, as in the old covenant Jewish sacrifices. Christ miraculously changes the nature of the bread and wine into his Body and Blood which he sacrificed for the salvation of the world.