Amen means yes! I believe it! If we wanted to get to the youths, we could say that amen is the “heck, yes” from Napoleon Dynamite. Throughout the history of the Mass, the most common words that were spoken together by the whole congregation would be “Lord, have mercy” and “Amen.”
Amen is an assent. It is an assent to the prayer that was begun with the faithful, taken up by the priest, and returned to the faithful for their assent in the prayer to God. Even though Christ’s faithful, the laity, don’t pray any of the other words of the Eucharistic Prayer, they have united themselves to the prayer that the priest has continued to pray that they entered into during the Preface. They are not silent spectators—they enter into the prayer with the fullness of their hearts, minds, and souls. They have united their prayer to Christ and to the words that the priest is praying. Before the Second Vatican Council, Pope Pius XII wrote an encyclical called Mediator Dei in 1947 on the Sacred Liturgy. In it he wrote:
“The cooperation of the faithful is required so that sinners may be individually purified in the blood of the Lamb. For though, speaking generally, Christ reconciled by His painful death the whole human race with the Father, He wished that all should approach and be drawn to His cross, especially by means of the sacraments and the eucharistic sacrifice, to obtain the salutary fruits produced by Him upon it. Through this active and individual participation, the members of the Mystical Body not only become daily more like to their divine Head, but the life flowing from the Head is imparted to the members, so that we can each repeat the words of St. Paul, ‘With Christ I am nailed to the cross: I live, now not I, but Christ liveth in me.’ We have already explained sufficiently and of set purpose on another occasion, that Jesus Christ ‘when dying on the cross, bestowed upon His Church, as a completely gratuitous gift, the immense treasure of the redemption. But when it is a question of distributing this treasure, He not only commits the work of sanctification to His Immaculate Spouse, but also wishes that, to a certain extent, sanctity should derive from her activity.’
“…It is, therefore, desirable, Venerable Brethren, that all the faithful should be aware that to participate in the eucharistic sacrifice is their chief duty and supreme dignity, and that not in an inert and negligent fashion, giving way to distractions and day-dreaming, but with such earnestness and concentration that they may be united as closely as possible with the High Priest, according to the Apostle, ‘Let this mind be in you which was also in Christ Jesus.’ And together with Him and through Him let them make their oblation, and in union with Him let them offer up themselves.
“..Now the faithful participate in the oblation, understood in this limited sense, after their own fashion and in a twofold manner, namely, because they not only offer the sacrifice by the hands of the priest, but also, to a certain extent, in union with him. It is by reason of this participation that the offering made by the people is also included in liturgical worship.”
– Mediator Dei, 78, 80, 92.
This is the Amen where we say, “yes, everything that was just offered, it is mine, too, in communion with the whole faithful of Christ throughout the world, and here at this altar, Amen!
For further reading see:
Pius XII, Mediator Dei, 1947
Why Is Participation at Mass So Difficult?!”