To understand the relationship between the cross and the Mass, we must understand Christianity. Christianity, as Fulton Sheen has explained, is not man making a quest for God, but God making a quest for man. This was made explicit when Our Blessed Lord became flesh. Many have lost the significance of the Incarnation. Fulton Sheen gave the best explanation:

"Suppose you were saddened by the way dogs were treated, beaten by strangers, starved, and driven from the company of men. To teach mankind to love dogs, further suppose that you divested your body and put your soul into the body of a dog. That would mean that inside the organism of a dog was an intellect capable of knowing God and a will capable of loving Him. Suppose that when you took on the form and habit of a dog, you resolved never to transcend the limitations of that animal organism. Though you had a mind that could scan the finite, you would never speak, you would not utter a word but would limit yourself to a bark. Though you were an artist, you would not use a brush to create. Second, suppose you resolved to subject yourself only to the companionship of other dogs, sharing their lives just in an effort to try and help them in virtue of your superior mind. That would indeed be an act of humility and a humiliation, particularly if you died defending the animals whose nature you embraced in order to save." [1]

This was the kind of humility Our Blessed Lord had to go through. One must always note the significance of what Our Blessed Lord has done, especially at Mass. Think of the humility Our Blessed Lord had to go through so that we may partake the sacrifice of the Cross. In order for us to partake of the cross, He would have to become present in a form of bread and wine. The question then would arise, how could the Mass be the same sacrifice as the cross without re-sacrificing Jesus?

To answer this question, we must remember that God is always making a quest for man and we must know the importance of a sacrifice. An animal sacrifice in the Old Testament meant many things: recognition of God's sovereignty over creation -- man acknowledged this fact by giving back to God what is ultimately His. Thus, sacrifice praised God from Whom all blessings flow. An act of thanks served as a way of solemnly sealing an agreement or oath, a covenant before God (Gen 21:22-32). The person offering sacrifice recognized that his sins deserved death; he offered the animal's life in place of his own. [2]

The sacrifice of the cross means all four of these. It made recognition of God's sovereignty over creation by showing that God conquers all things including death. This is also an important point: that the cross and the resurrection cannot be separated for if there is no cross, there would be no resurrection, and if there is no resurrection, then the cross is meaningless. The cross and the resurrection are progressive and affect us now. The cross would then affect us by the Mass.

The reason why there is the Mass is because the cross must be applied to us. Catholic theologian Dr. Ludwig Ott wrote:

"While the Sacrifice on the Cross is an absolute sacrifice, as it is neither the commemoration of a past sacrifice nor the archetype of a future sacrifice, the Sacrifice of the Mass is a relative sacrifice, as it is essentially linked to the Sacrifice on the Cross. The Council of Trent teaches: Christ left a visible Sacrifice to His Church: in which that bloody sacrifice which was once offered on the Cross should be made present, its memory preserved to the end of the world, and its salvation-bringing power applied to the forgiveness of the sins which are daily committed by us." [3]

The Mass then, is the sacrifice of the Mystical Body of Christ, and is one with Calvary, which was the sacrifice of the physical body of Christ. [4]  It is this reason why Our Blessed Lord founded the Church. Christ founded the Church in order to continue His work of redemption for all time. [5]  How can the Mass be the same sacrifice of the Cross? Fr. Mario Romero gave the best explanation I have heard:

"Suppose I took a black marker and made one dot on a rubber band. That one dot would be present at one point on that rubber band. Suppose, now, that I stretched that rubber band. What would happen to that one dot? It would be the same one dot but it would now become present at various points on the rubber band. The same holds true with the sacrifice of the Mass. It was done once by Jesus 2,000 years ago but is made present to Christians throughout the ages as a way to continually renew our covenant with Jesus." [6]

The Cross was in Jerusalem as a space, and it was two thousand years ago in time; the Mass is the same Cross made actual throughout space and time. The Cross purchases redemption; the Mass applies it. [7]  Our Blessed Lord did not want us to just 'remember' the Cross, but to live it by partaking of it. Sheen has said, 'As man died spiritually by physically eating in the Garden of Eden, so he would live again spiritually through eating the fruit of the Tree of Life.' [8]  The Mass is Calvary recalled, renewed, and applied. [9]  The same Cross is offered up (made present) again and Christ acts as a High Priest through His Mystical Body just as He did at the Last Supper in His Physical Body. Robert Sungenis notes:

"In effect, Christ does in a perfect way what Moses did in an imperfect way, appealing to God on our behalf. In His act, He brings to God, moment by moment, the memory of His once-for-all supreme sacrifice on the Cross and seeks to propitiate God's wrath against sin