The Constitution on the Sacred Liturgy (Sacrosanctum Concilium)
On December 4, 1963 the council issued The Constitution on the Sacred Liturgy. The document has four goals for the reform and promotion of the liturgy: to invigorate the lives of the faithful, to adapt more suitably to the needs of our time, to promote whatever serves to bring unity among Christians, and to strengthen whatever serves to invite all people into the Church.
“To accomplish so great a work Christ is always present in his church, especially in liturgical celebrations. He is present in the sacrifice of the Mass both in the person of his minister, “the same now offering through the ministry of priests, who formerly offered himself on the cross,” and most of all in the Eucharistic species.
By his power he is present in the sacraments so that when anybody baptizes it is really Christ himself who baptizes. He is present in his world since it is he himself who speaks when the holy scriptures are read in church.
Lastly, he is present when the church prays and sings, for he has promised, “where two or three are gathered together in my name there am I in the midst of them.” [SC7]
“The liturgy is the summit toward which the activity of the Church is directed; it is also the source from which all its power flows.”[SC10]
“It is very much the wish of the church that all the faithful should be led to take that full, conscious and active part in liturgical celebrations which is demanded by the very nature of the liturgy, and to which the Christian people, “a chosen race, a royal priesthood, a holy nation, a redeemed people” have right and to which they are bound by reason of their Baptism.” [SC14]
“Liturgical services are not private functions but are celebrations of the church which is “the sacrament of unity,” namely, the holy people united and organized under their bishops” [SC26]
“For in the liturgy God speaks to his people, Christ is still proclaiming his gospel, and the people respond to God both in song and prayer.” [SC33]
You can read the documents of Vatican II at: Documents of the Vatican II Council