The Most Sacred Mystery of The Eucharist (Part I)
At the Last Supper, on the night when He was betrayed, our Saviour instituted the Eucharistic sacrifice of His Body and Blood. He did this in order to perpetuate the sacrifice of the Cross throughout the centuries w1til He should come again, and so to entrust to His beloved spouse, the Church, a memorial of His death and resurrection: a sacrament of love, a sign of unity, a bond of charity (St. Augustine, Tractatus in Ioannem, VI, n. 13.), a paschal banquet in which Christ is eaten, the mind is filled with grace, and a pledge of future glory is given to us (Roman Breviary, feast of Corpus Christi, Second Vespers, antiphon to the Magnificat).
The Church, therefore, earnestly desires that Christ's faithful, when present at this mystery of faith, should not be there as strangers or silent spectators; on the contrary, through a good understanding of the rites and prayers they should take part in the sacred action conscious of what they are doing, with devotion and full collaboration. They should be instructed by God's word and be nourished at the table of the Lord's body; they should give thanks to God; by offering the Immaculate Victim, not only through the hands of the priest, but also with him, they should learn also to offer themselves; through Christ the Mediator (St. Cyril of Alexandria, Commentary on the Gospel of John, book XI, chap. XI-XTI: Migne, Patrologia Graeca, 74, 557-564), they should be drawn day by day into ever more perfect union with God and with each other, so that finally God may be all in all.
-Constitution on the Sacred Liturgy, Sacrosanctum Concilium # 47- 48