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. 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.
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. 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.
Finally, this catalogue of specialized roles might give the impression that those who are not exercising one of these roles are free to sit back passively and simply let the liturgy happen around them. Nothing could be further from the truth. Those who come together for liturgy do not have the luxury of acting passively, waiting for all to be done for them. The liturgy is not only their right, it is their duty, their responsibility and that responsibility includes full engagement throughout the liturgical celebration.
The liturgy, then, is about the action of God's own people each with different offices and roles, each office and role, from that of bishop and priest to that of usher and sacristan, one of service, not of privilege, a mirror of Christ who washed the feet of his disciples and instructed his followers to imitate his example of service. (U.S. Conference of Catholic Bishops, July 2020)