The Mass as the Central Act of Worship in the Catholic Church:
Catholics worship God in a variety of ways, but the chief act of corporate or communal worship is the Liturgy of the Eucharist. In the Eastern churches, Catholic and Orthodox, this is known as the Divine Liturgy; in the West, it is known as the Mass, an English word derived from the Latin text of the priest's dismissal of the congregation at the end of the liturgy ("Ite, missa est."). Throughout the centuries, the liturgy of the Church has taken a variety of regional and historical forms, but one thing has remained constant: The Mass has always been the central form of Catholic worship.
The Mass: An Ancient Practice:
As far back as the Acts of the Apostles and Saint Paul's epistles, we find descriptions of the Christian community gathering to celebrate the Lord's Supper, the Eucharist. In the catacombs in Rome, the tombs of martyrs were used as altars for the celebration of the earliest forms of the Mass, making the tie between the sacrifice of Christ on the Cross, its re-presentation in the Mass, and the strengthening of the faith of Christians explicit.
The Mass as "Unbloody Sacrifice":
Very early on, the Church saw the Mass as a mystical reality in which the sacrifice of Christ on the Cross is renewed. Responding to Protestant sects who denied that the Eucharist is anything more than a memorial, the Council of Trent (1545-63) declared that "The same Christ who offered himself once in a bloody manner on the altar of the cross, is present and offered in an unbloody manner" in the Mass.
This does not mean, as some critics of Catholicism claim, that the Church teaches that, in the Mass, we sacrifice Christ again. Rather, the original sacrifice of Christ on the Cross is presented to us once more.
The Mass as a Re-presentation of the Crucifixion:
This re-presentation, as Fr. John Hardon notes in his Pocket Catholic Dictionary, "means that because Christ is really present in his humanity, in heaven and on the altar, he is capable now as he was on Good Friday of freely offering himself to the Father." This understanding of the Mass hinges on the Catholic doctrine of the Real Presence of Christ in the Eucharist. When the bread and wine become the Body and Blood of Jesus Christ, Christ is truly present on the altar. If the bread and wine remained merely symbols, the Mass could still be a memorial of the Last Supper, but not a re-presentation of the Crucifixion.
The Mass as Memorial and Sacred Banquet:
While the Mass is more than a memorial, it is still a memorial as well as a sacrifice. The Mass is the Church's way of fulfilling Christ's command, at the Last Supper, to "Do this in remembrance of Me." As a memorial of the Last Supper, the Mass is also a sacred banquet, in which the faithful participate both through their presence and their role in the liturgy and through the reception of Holy Communion, the Body and Blood of Christ.
The Mass as an Application of the Merits of Christ:
"Christ," Father Hardon writes, "won for the world all the graces it needs for salvation and sanctification." In other words, in His Sacrifice on the Cross, Christ reversed Adam's sin. In order for us to see the effects of that reversal, however, we must accept Christ's offer of salvation and grow in sanctification. Our participation in the Mass, and our frequent reception of Holy Communion, brings us the grace that Christ merited for the world through His unselfish Sacrifice on the Cross.