1 The Mystery of Faith, that is, the ineffable gift of the Eucharist that the Catholic Church received from Christ, her Spouse, as a pledge of His immense love, is something that she has always devoutly guarded as her most precious treasure, and during the Second Vatican Council she professed her faith and veneration in a new and solemn declaration. In dealing with the restoration of the sacred liturgy, the Fathers of the Council were led by their pastoral concern for the whole Church to regard it as a matter of highest importance to urge the faithful to participate actively, with undivided faith and the utmost devotion, in the celebration of this Most Holy Mystery, to offer it to God along with the priest as a sacrifice for their own salvation and that of the whole world, and to use it as spiritual nourishment.
Source: Mysterium Fidei (Vatican.va)