In our Lord’s public life, through His preaching, healing and miracles, He proclaimed the goodness and mercy of God towards humanity. This mission reached its culmination at Calvary with the Lord’s sacrificial death on the Cross. The sacrifice of Calvary was mysteriously anticipated during the Last Supper, the night before He died when our Lord, sharing with the Twelve the bread and wine, transformed them into His body and His blood, which shortly after He would offer really and truly as the Lamb of God.
The Eucharist is the memorial of the Passion, Death, Resurrection and Ascension of Jesus Christ, of His love to the end for each one of us. He willed the same sacrifice to be entrusted to the Church down through the centuries, until the end of the world. However, it is not just a memory of a past event; rather the “memorial” actualizes, and makes present that event, so as to reproduce its saving power and efficacy. Thus, "the sacrifice that Christ offered to the Father, once and for all, on the Cross in favor of humanity, is rendered present and actual" (Catechism of the Catholic Church, No. 280).
At Mass we enter into a reality that preceded us, also a reality which embraces heaven and earth, and the past, future and the present. The Eucharist transforms a group of people into the Church: the Eucharist makes the Church. At every single Mass the greatest miracle on earth occurs when bread and wine becomes Christ at the consecration by the power of God the Holy Spirit.
So the Mass is the perfect form of worship of God given to us by Christ at the Last Supper. During the Mass we pray not only through our soul but our body, which is why we bow, kneel, and stand at certain points.
First and foremost, the Mass is a Sacrifice and secondly it is a Sacred Meal. In the Old Testament, people would frequently offer animal sacrifices to God in atonement for their sins as was prescribed by the Law of Moses. These sacrifices were offered by a priest; the purpose of a priest is to offer sacrifice to God.
We offer to God the only true Sacrifice, now the only acceptable sacrifice - Jesus Christ Himself. Our Lord is the Lamb of God because He was the sacrifice that paid the price for all of the sins and that of the whole world. All the sins of all humanity, past, present and future were paid for by the blood of the Lamb at Calvary.
Christ came and died for all of our sins, He paid the price for our sins, He redeemed us and brought us back from the power of the devil. Because of our Lord’s death and Resurrection, all of us have the chance to enter Heaven. We remember this at every Mass.
The Mass is also a sacred Meal. We gather as the family of God around the table or the altar and share in the meal actually receiving food and drink: Christ's Body and Blood. Even in the Old Testament sacrifices, the culmination of the sacrifice was when the people had to eat the sacrifice. At Mass, we consume the Lamb of God and become what we receive. We always need to be in the state of grace, in communion with God to receive Him properly.