5 “ Mysterium fidei! - The Mystery of Faith!”. When the priest recites or chants these words, all present acclaim: “We announce your death, O Lord, and we proclaim your resurrection, until you come in glory”. In these or similar words the Church, while pointing to Christ in the mystery of his passion, also reveals her own mystery : Ecclesia de Eucharistia . By the gift of the Holy Spirit at Pentecost the Church was born and set out upon the pathways of the world, yet a decisive moment in her taking shape was certainly the institution of the Eucharist in the Upper Room. Her foundation and wellspring is the whole Triduum paschale , but this is as it were gathered up, foreshadowed and “concentrated' for ever in the gift of the Eucharist. In this gift Jesus Christ entrusted to his Church the perennial making present of the paschal mystery. With it he brought about a mysterious “oneness in time” between that Triduum and the passage of the centuries. The thought of this leads us to profound amazement and gratitude. In the paschal event and the Eucharist which makes it present throughout the centuries, there is a truly enormous “capacity” which embraces all of history as the recipient of the grace of the redemption. This amazement should always fill the Church assembled for the celebration of the Eucharist. But in a special way it should fill the minister of the Eucharist. For it is he who, by the authority given him in the sacrament of priestly ordination, effects the consecration. It is he who says with the power coming to him from Christ in the Upper Room: “This is my body which will be given up for you This is the cup of my blood, poured out for you...”. The priest says these words, or rather he puts his voice at the disposal of the One who spoke these words in the Upper Room and who desires that they should be repeated in every generation by all those who in the Church ministerially share in his priesthood.
Source: Ecclesia de Eucharistia (Vatican.va)