"Christ is always present in his Church, especially in its liturgical celebrations.  He is present in the sacrifice of the Mass, not only in the person of the minister, 'the same now offering, through the ministry of priests, who formerly offered himself on the cross', but especially under the Eucharistic elements.  By this power he is present in the sacraments, so that when a man baptizes it is really Christ himself who baptizes.  He is present in the word, since it is he himself who speaks when the holy Scriptures are read in the Church.  He is present, lastly, when the Church prays and sings, for he promised: 'Where two or three are gathered together in my name, there am I in the midst of them'. (Mt. 18:20)

The liturgy is the summit toward which the activity of the Church is directed; at the same time it is the fount from which all the Church's power flows."

(Constitution on the Sacred Liturgy # 7 and # 10)

Information on the Changes to the Mass