The mystery of Easter is the center and culmination of the entire liturgical year. Every mystery of the faith which we celebrate throughout the whole year is directed and ordered to the resurrection. Without the resurrection, "our faith would be in vain," says St. Paul. The resurrection is the definitive enabling cause of our salvation and union with God, the flipside of the passion and death. Christ suffered and died, not only that He might make satisfaction for our sins, but that He might conquer the death of sin by rising again on the third day. The passion of Christ is incomplete without the resurrection.
The essential life of the Christian is a participation in the life of Christ Himself. Just as the divine person of Christ participated in human nature, so must we humans participate in divine nature, through our participation in the life of Christ Himself. Grace makes us sons of God and partakers of the divine nature. It elevates us to a spiritual adoption by God the Father, of Whom Christ said "no one comes to the Father except through Me." Therefore our spiritual adoption is only by participation in Christ's own life. The resurrection too, then, must become ours. Christ's resurrection becomes our resurrection.
Now we participate in Christ's resurrection through various means. First and foremost is the sacrament of Baptism, which the fathers and doctors of the Church teach us is a participation in "Christ dying and rising." For by Baptism, we die to sin and rise to God, regenerated with a new life of the soul. Baptism by immersion in an especially vivid manner recreates the image of being buried (immersed under water) and rising (emerging from the water). Secondly to Baptism, our participation in the resurrection occurs through the liturgical celebration of the resurrection, which is repeated every year on the feast of Easter, and also in various other ways throughout the year (every Sunday, every octave, etc). The constant renewal of this mystery within the liturgical cycle allows us to contemplate the mystery by a participation in the wisdom of God, thereby affecting our assimilation to God through the mystery itself. The resurrection thus literally becomes ours, or the promise of it. The fruits of Baptism itself gain more increase in this way, and our final resurrection to eternal life is better ensured. The liturgy, on the foundation of the sacraments, especially Baptism, is the primary means of our participating in eternal life itself, or its promise, in the virtue of hope.
The liturgy of the Easter Vigil makes these mysteries abundantly clear. The Easter Vigil is traditionally also a celebration of the sacrament of Baptism. The Baptismal font itself is blessed, and the catechumens are baptized this night. The readings of the Easter Vigil - twelve of them in the old rite - are a majestic meditation on the mystery of Baptism and the resurrection, prefigured and foretold in the Old Testament. Genesis, for example, gives us the accounts of the creation and the deluge, signifying the re-creation and purgation that occurs in Baptism. Abraham consents to sacrifice his son, who is nonetheless preserved from the hold of death. In Exodus, the Israelites pass dryshod through the Red Sea, emerging victorious from their Egyptian bondage, a symbol of the free people of the resurrection and regeneration that is Baptism; and yet the sea swallows and drowns Egyptian king and soldiers, who signify the forces of sin and death. A striking passage in Ezekiel relates the prophets vision of the valley of the dry bones, which rise to new life at the breath of God. The Paschal Lamb appears in another reading from Exodus. The whole liturgy is permeated with an awareness of the Baptismal significance of the resurrection, the mystery of Easter.
St. Augustine speaks of the two regenerations or resurrections. The first is the regeneration of the soul, and so it is the promise of eternal life; one who perseveres in the graces of Baptism will receive the fulfillment of the promise, as well as the second resurrection, of the body, to salvation. But those who do not persevere in the graces of Baptism will fall away from the first resurrection, which is of the soul, but will suffer the second resurrection, of the body, unto judgment and eternal damnation. The people of the City of God participate in the first resurrection, and by living the full life of that City while on pilgrimage on this earth, they better ensure their own perseverance in the first resurrection, that they might enjoy its fulfillment in eternity. The earthly incarnation of the City of God is of course the Church, whose primary acts are the sacraments and the liturgy. Thus it is by our participation in those primary acts that we participate in the first resurrection, and ensure our second resurrection unto eternal life.