Dear Parish Faithful & Friends in Christ,
That simplest of liturgically-used words - Amen - is packed with profound meaning. That means that this word must be said with care, as well as with faith and a deep and abiding sense of its significance. Here is further commentary from Hieromonk Gregorios, as he interweaves the thought of the Fathers with his own reflection:
The faithful set their seal on the doxology pronounced by the priest by singing Amen. (Amen is a Hebrew word meaning 'indeed', 'let it be so'. St. Jerome compares the singing of the Amen with thunder from heaven). With this response, the faithful accept the truth expressed in the celebrant's proclamation, and pray that they may taste the good things of the Kingdom of the Trinity.
The faithful 'conclude with Amen, and thus making everything that the priest says their own.' (St. Nicholas Cabasilas). This ending to every pronouncement of the priest signifies that what 'is lacking in the perfection of the priests is completed by the action of the people, and God accepts the least with the greatest [Ps. 113:21] in one unity of spirit. For the congregation also believes that their prayers are accepted when they join them to the prayers of the priests.' (St. Cyril of Alexandria)
With the assent of the faithful, the priest's blessing ascends to the Altar above the heavens. The faithful actively participate in the Divine Liturgy, which at every moment confirms its name: it is a work of the people. (p. 109-110)
Every single person who is at a given service should always join in with the singing or chanting of the Amen. This can be done "loudly" or "softly." I included a few words about the Amen in my booklet on the Divine Liturgy:
We may or may not choose to sing but all of the faithful, without exception, should join in the singing/saying of the AMEN which is the seal of every prayer and is a significant liturgical action. This is especially true and crucial during the consecration of the Holy Gifts, when we pray that by the power and grace of the Holy Spirit, the bread and wine offered on the altar will indeed become the Body and Blood of Christ. Each AMEN over the paten and chalice, and our triple AMEN, AMEN, AMEN, when the priest prays that this change be made "by the Holy Spirit" needs to ring out loudly and clearly as our affirmation of faith and thanksgiving in the presence of the Holy Trinity.
I remember con-celebrating at the Holy Trinity cathedral one Sunday morning in Boston many years ago, when the triple Amen from the faithful was so loud and powerful that I was initially startled and it felt as if "the walls were shaking." The Amen can have that effect when pronounced by a large gathering of the faithful, especially within the context of the liturgical consecration of the Gifts. But again, the volume is not as important as the faith and commitment to God's Kingdom coming from our minds and hearts when we seal our prayers with that sacred word Amen!