Following the 'Mystery of Faith,' the priest continues with the Eucharistic Prayer. The inner organization of the Eucharistic Prayers and their various elements is quite splendid, but that is a topic for another time. It is important to note, however, that the entire Eucharistic Prayer is addressed to God the Father. This great prayer culminates in the saying of the doxology and 'The Great Amen.'
"Through him, with him, and in him, O God, almighty Father, in the unity of the Holy Spirit, all glory and honor is yours, forever and ever."
The first part (said by the priest) is the doxology, which means an 'account' (-oxology) of 'Glory' (doxa-). We have doxologies all over the place in Catholicism. The "Glory Be" prayer is a doxology. The 'Gloria' earlier on in the Mass is a doxology. However, there is something special about the doxology that follows the Eucharistic Prayer.
Consider what has happened in the Mass up until now. We gathered our various prayers at the offertory and, along with the physical gifts of bread and wine, placed them at the altar. The priest, who stands in persona Christi to perform the priestly function of Jesus Christ the Son, begins praying to God the Father, preparing to sacrifice these gifts. But the gifts are not worthy yet. Only one gift is worthy of heaven: Christ himself. And so the priest, by the power of the Holy Spirit, consecrates and transforms the simple offering of bread and wine into the very body and blood of Jesus Christ,
who willingly went to suffer and die for this very purpose: to offer Himself as a perfect Sacrifice to the Father. After the consecration is complete and the priest has concluded the Eucharistic Prayer, we have the doxology. We finally have all the elements needed for the sacrifice of the Mass. We have an adequate offering (the body and blood of Christ.) We have an adequate priest to offer it (the priest acting in persona Chrsti) and we have annunciated all of the reasons for this Sacrifice (our salvation, the salvation of the whole Church, and the various particular petitions of that Mass.) And so, the priest lifts the gifts to heaven and, annunciating the involvement of all three person's of the Trinity in this moment, offers Jesus Christ as a sacrifice for the glory of God and for us.
This action is done "through him, with him, and in him." Jesus is the willing victim of the sacrifice. He is the high priest who offers the victim. And He is the one whose perfection and purity makes our own sacrifices acceptable by their association with Him. We are utterly dependent upon the Grace of Christ Jesus, who so wonderfully performed the incredible economy of salvation for us. He is all the parts of that economy. We are the unmerited recipients.
So, the priest is saying and doing all this. He lifts the gifts (with the help of the deacon), chants the words, etc. But as we have heard over and over again up until now in various articles in the Liturgy Corner, Mass is not a spectator sport. Ok, at some points it is, even for the priest (listening to the readings for example.) But even at the moments where no external activity is called for, internal participation is necessary. All through the Eucharistic Prayer, the congregation has been patiently listening, trying to fend off various distractions, just trying to be attentive to what the priest is doing on their behalf. And now comes the climax. The whole congregation is there, kneeling at the feet of God the Father, beseeching His mercy, not by any merit of their own, but by the merits of the perfect Sacrifice of Jesus Christ. Their 'spokesperson' has said his piece and done his work. And as he makes the final offering, the congregation raises their voices and says 'The Great Amen.' It is so hard to explain all that is meant by the word 'Amen.' One decently adequate translation is 'I assent with all my being.'
I like to mentally step back and imagine the spiritual vision of this moment. We are in the throne room of God to petition Him. Jesus stands at our head and speaks on our behalf, offering Himself as the Sacrifice, and at the pinnacle of this event the people gathered behind Him give their assent to what their leader has said and done. We say, "He speaks and acts for all of us."