The Catholic Church has always devoutly guarded as a most precious treasure the mystery of faith, that is, the ineffable gift of the Eucharist which she received from Christ her Spouse as a pledge of His immense love, and during the Second Vatican Council in a new and solemn demonstration she professed her faith and veneration for this mystery. When dealing with the restoration of the sacred liturgy, the Fathers of the council, by reason of their pastoral concern for the whole Church, considered it of the highest importance to exhort the faithful to participate actively with sound faith and with the utmost devotion in the celebration of this Most Holy Mystery, to offer it with the priest to God as a sacrifice for their own salvation and for that of the whole world, and to find in it spiritual nourishment.
For if the sacred liturgy holds the first place in the life of the Church, the Eucharistic Mystery stands at the heart and center of the liturgy, since it is the font of life by which we are cleansed and strengthened to lie not for ourselves but for God, and to be united in love among ourselves. To make evident the indissoluble bond which exists between faith and devotion, the Fathers of the council, confirming the doctrine which the Church has always held and taught and which was solemnly defined by the Council of Trent, determine to introduce their treatise on the Most Holy Mystery of the Eucharist with the following summary of truths:< At the Last Supper, on the night He was handed over, Our Lord instituted the Eucharistic Sacrifice of His Body and Blood, to perpetuate the sacrifice of the cross throughout the ages until He should come, and thus entrust to the Church, His beloved spouse, the memorial of His death and resurrection: a sacrament of devotion, a sign of unity, a bond of charity, a paschal banquet in which Christ is received, the soul is filled with grace and there is given to us the pledge of future glory.
In these words are highlighted both the sacrifice, which pertains to the essence of the Mass which is celebrated daily, and the sacrament in which the faithful participate in Holy Communion by eating the Flesh of Christ and drinking His Blood, receiving both grace, the beginning of eternal life, and the medicine of immortality. According to the words of Our Lord: The man who eats my flesh and drinks my blood enjoys eternal life, and I will raise him up at the last day.
Therefore we earnestly hope that the restored sacred liturgy will bring forth abundant fruits of eucharistic devotion, so that the Holy Church, under this saving sign of piety, may make daily progress toward perfect unity and may invite all Christians to a unity of faith and of love, drawing them gently, thanks to the action of divine grace. (Papal Encyclical, Mysterium Fidei, 1965).