Christ, our paschal lamb, was sacrificed. The verse for the Alleluia of Mass during the day on Easter Sunday gives the sacrifice of Christ as the cause of our joy. It is by sacrifice that Christ won His victory over sin and death. What is sacrifice? It is an outward, sensible sign of interior self-giving love. Sin is refusal of love and gift. God wanted to give everything to us; He wanted to give Himself to us. But this gift cannot be received and possessed as a private good; it can only be had as a common good. And a common good is possessed when one gives oneself to it. To receive the gift of God is to give oneself to God. And to give oneself to God is to receive oneself from Him. Hence, as Gaudium et Spes puts it, “man cannot fully find himself except through a sincere gift of himself.”
Sin is the refusal to give oneself to the common good, and hence to receive it. Sin is to prefer the private good, that one can have of oneself, to the common good that requires transcendence of the self and communion with others. John of St Thomas (in a text cited by Charles de Koninck in On the Primacy of the Common Good), argues that the fallen angels fell because they did not want to possess beatitude as a common good; they preferred an inferior good which was private and not shared with others, and which therefore did not require self-transcendence:
The parable in Luke 14 bears on this point, the parable about the man who prepared a great dinner and called many and when he called the invited began to excuse themselves. Perhaps they excused themselves from coming to that dinner because it was great and for many, feeling contempt for sharing it with so great a number, and rather chose their own advantages even though they were by far inferior, namely, of the natural order, one because he bought a house, another a yoke of oxen, another because he was marrying, each of them bringing forward his own excuse and private good because it was his own, rejecting the dinner because it was great and common to many. This is most proper to the spirit of pride.
Sacrifice is the antidote to such pride. In sacrifice one gives up a private good to God (eg. by pouring out wine, burning grain, killing cattle) in order to signify that one wants to give oneself God, that one loves Him as the common good, and is willing to give up one’s private good for His sake.
Christ’s sacrifice begins when He takes on our condition as creatures. This is the opposite of sin; the sinner wants to be his own chief good, the final end of his own life, but Christ, does precisely the opposite: “though He was in the form of God, He did not count equality with God a thing to be grasped, but emptied himself, taking the form of a servant, being born in the likeness of men” (Phil 2:6-7). But His sacrifice is completed when He gives away His life on the Cross: “and being found in human form he humbled himself and became obedient unto death, even death on a cross” (Phil 2:8). This sacrifice is the most perfect imaginable expression of self-donation to God as the common good— He does not give up some private possession, but His very life.
But then God shows that to give to Him is to receive from Him: “Therefore God has highly exalted him and bestowed on him the name which is above every name” (Phil 2:9). To give one’s life to God is to receive God’s life as one’s own life— not as a private possession, but as a common good that is shared. Christ receives in His exaltation not only His own life, but also our life— the eternal, blessed life that we receive as members of His body.