When the Church comes together in the liturgical assembly to celebrate the Mass, or any other sacrament, her members do not gather simply as a crowd, as an amorphous, undifferentiated group of people. They gather in a variety of ministries and roles. If we are to understand the significance of these ministries and roles, we must begin with Baptism, for only one who through Baptism has been given a share in the priesthood of Christ is capable of participating in the public worship which is the liturgy of the Church. In fact, the Constitution on the Sacred Liturgy of the Second Vatican Council tells us that participation in the liturgy is the right and duty of all the baptized.

The first qualification, then, for any participant in the celebration of the Eucharist is that he or she has been baptized into the Body of Christ, the Church. Each time the members of the Church gather to worship, they do so because their baptism demands this of them. While all share in the priesthood of Christ which delegates them to worship, some members of the Church are called by God to serve in the ministerial priesthood as bishops and priests. Bishops and priests are privileged to act in the liturgy in the very person of Christ, on behalf of his people, pronouncing the most sacred prayers of our faith, presiding over the celebration of the sacred mysteries, explaining God's Word and feeding God's people on the body and blood of Christ. A bishop has the added responsibility of being the chief shepherd, the principal liturgist of his diocese and in that role is the successor of the Apostles. Others by God's grace are ordained to the ministry of deacon. In the celebration of the Mass deacons proclaim the Gospel and assist the bishop and priest in exercising their sacred duties.

In addition to the ordained ministries there are roles in the liturgy which are exercised by lay people who place their time and talent at the service of the liturgical assembly as acolytes (altar servers), lectors, extraordinary ministers of Holy Communion, cantors, choir members, instrumentalists, leaders of song and ushers. Others contribute their time and talent to planning and organizing the liturgy, to keeping the church and the vestments, vessels and appointments clean and well-ordered or to providing decorations that reflect the spirit of the liturgical feast or season.