The Mass is a sacrifice in the sense that when it takes place, Jesus Christ, through the bishop or priest celebrating the Mass, makes present sacramentally His saving, sacrificial death on the Cross by which He redeemed us from our sins. This Eucharistic sacrifice is the memorial of Christ’s redeeming death. The term memorial in this context is not simply a remembrance of past events; it is a making present in a sacramental manner the sacrifice of the Cross of Christ and His victory. The Eucharistic sacrifice is offered to adore and thank God, to pray for all our needs, and to gain pardon for our sins.

In this divine sacrifice which is made present in the Mass, especially in the Eucharistic Prayer, the same Christ who offered himself once in a bloody manner on the altar of the Cross offers himself in an unbloody manner. Present and effective, Christ’s sacrifice is applied to our lives.

The Mass is also the sacrifice of the Church. The ordained priest in the Mass links the Eucharistic consecration to the sacrifice of the Cross and to the Last Supper (cf. EE, no. 29), thus making it possible that the sacrifice of Christ becomes the sacrifice of all members of the Church. “The lives of the faithful, their praise, sufferings, prayer, and work, are united with those of Christ and with His total offering, and so acquire a new value” (CCC, no. 1368).

Jesus Christ shares with us His Body and Blood under the form of bread and wine. Thus the Mass is a sacred banquet that culminates in the reception of Holy Communion.

Holy Communion increases our union with Christ. Just as bodily food sustains our physical life, so Holy Communion nourishes our spiritual life. This Communion moves us away from sin, strengthening our moral resolve to avoid evil and turn ever more powerfully toward God. “The more we share the life of Christ and progress in his friendship, the more difficult it is to break away from him by mortal sin” (CCC, no. 1395).

By the power of the Holy Spirit, Christ is present in the proclamation of God’s Word, in the Eucharistic assembly, in the person of the priest, but above all and in a wholly unique manner in the Eucharist.

Through participation in the Eucharist, we also participate in the Paschal Mystery of Christ, that is, in His dying and rising, which is made present for us in the Eucharistic sacrifice. This participation in the Paschal Mystery of Christ reaches its consummation when we receive His Body and Blood in Holy Communion. Christ’s victory and triumph over death is then made present in the lives of those who participate in the Eucharist.

Communion with the Body of Christ preserves, increases, and renews the lift of grace received at Baptism.

Holy Communion separates us from sin. Our love of God is intensified and therefore our disordered attachments are weakened and even broken. Divine love wipes away venial sins.

Holy Communion offers us strength, called grace, to preserve us from mortal sin. By deepening our friendship with Christ, this Sacrament makes it more difficult for us to break our union with him by mortal sin.

When we receive Communion, we need to remember that we are not changing Christ into ourselves. Jesus is transforming us into Himself. This requires a proper understanding of the Real Presence of Jesus under the appearance of bread and wine. It is not simply a symbol that merely points to Jesus. Nor is Christ’s presence just a projection on our part in the sense that we make Him present when we receive Him. As Pope Benedict XVI told the young people gathered for the Twentieth World Youth Day:

“The Body and Blood of Christ are given to us so that we ourselves will be transformed in our turn.
We are to become the Body of Christ, His own Flesh and Blood.
  We all eat the one bread, and this means that we ourselves become one.
In this way, adoration, as we said earlier, becomes union. God no longer simply stands before us
as the One who is totally Other. He is within us, and we are in Him.
His dynamic enters into us then seeks to spread outwards to others until it fills the world,
so that His love can truly become dominant measure of the world.”
(Benedict XVI, Homily at Marienfeld, Twentieth World Youth Day [August 21, 2015])

The consecrated bread has become Christ’s Body. The consecrated wine has become Christ’s Blood. Jesus Christ is substantially present in a way that is entirely unique. This happens by the power of the Holy Spirit though the ministry of the priest’s or bishop’s acting in the person of Christ during the Eucharistic Prayer. At Mass, when we are offered the Host and hear the statement “The Body of Christ,” we answer, “Amen,” that is, “Yes, I believe.”