What is the Liturgy?               

The word ‘liturgy’ means ‘public work’ or ‘service in the name or on behalf of the people’. In Christian tradition, it means the participation of the People of God in the work of God himself, in worship, in proclamation of the Gospel and in charity. Through the sacred liturgy, Christ, our High Priest, continues the work of our redemption through, with and in his Church. The liturgy is summed up in the doxology:

Through him, and with him, and in him,
O God, almighty Father
in the unity of the Holy Spirit
all glory and honour is yours
for ever and ever.

‘The liturgy is the work of the whole Christ, head and body. Our high priest celebrates it unceasingly in the heavenly liturgy, with the holy Mother of God, the apostles, all the saints and the multitude of those who have already entered the kingdom’ (Catechism of the Catholic Church 1187)

In the sacramental liturgy here on earth, in which God is revealed in signs and symbols, we celebrate the heavenly liturgy. As public worship, it is the whole of the community of the Body of Christ which celebrates:

Liturgical services are not private functions but are celebrations of the Church which is the ‘sacrament of unity’, namely, the holy people united and organised under the authority of the bishops. Therefore, liturgical services pertain to the whole Body of the Church. They manifest it, and have effects upon it. But they touch individual members of the Church in different ways, depending on their orders, their role in the liturgical services, and their actual participation in them’. (Sacrosanctum Concilium 27)

What is a Sacrament?

A sacrament is ‘an outward sign of an inward grace’. Grace is ‘favour, the free and undeserved help that God gives us to respond to his call to become children of God, adoptive sons, partakers of the divine nature and of eternal life’ (CCC 1996). Grace is participation in the life of the Holy Trinity, the supernatural vocation to eternal life, which depends wholly on God’s gratuitous initiative:

• Sanctifying grace is given in the sacraments; it is a habitual gift, a stable and supernatural disposition that perfects the soul itself to enable it to live with God, to act by his love.

• Habitual grace is the permanent disposition to live and act in the keeping with God’s call.

• Actual graces are God’s interventions, whether at the beginning of conversion or in the course of sanctification.

• Special graces are the various charisms. Prevenient grace is the gift of God which enables us to respond to his original call before we receive the sacraments; it is needed to arouse and sustain our cooperation in justification through faith and in sanctification through charity.
http://www.vatican.va/archive/ccc_css/archive/catechism/p2s2c1a1.htm