Amen is a word that came to English from Latin, which got it from Greek, which got it from Aramaic, which got it from Hebrew. It is difficult to translate this word directly, which is often a reason that words are borrowed from other languages (i.e., if there’s no direct way to translate this foreign word, just borrow it).
The specific Hebrew word amen (’amen ) appears to be derived from a related verb–’aman , which means “he confirmed, supported, or upheld.” This verb is also associated with the Hebrew word for truth (’emet ), which carries the idea of certainty or dependability (i.e., that which is true is that which is certain or dependable).
’Amen’ itself is an interjection used to agree with, affirm, approve, or emphasize something else that has been said. Thus when Jesus begins certain sayings by declaring “Amen, amen, I say to you . . . “ various Bible translations often render the “amen, amen” different ways. Because of the word’s association with the Hebrew terms for truth, the double amen is sometimes rendered “truly, truly” or “verily, verily.” Because of its association with the Hebrew terms for confirmation or dependability, one might also translate it “certainly, certainly” or “most assuredly.”
When one says “amen” in response to a prayer, it serves as an affirmation of agreement with the content of the prayer (cf. 1 Cor 14:16)—in which case it is sometimes translated “So be it” (cf. CCC 2856)—or as an expression of faith that God will hear and act on the prayer.
When a communicant says, “Amen,” to the words “The Body of Christ” when he or she receives the Eucharist at Mass, they are saying “Amen” to several realities. First, “Amen” to the reality of the real presence of Christ in the Eucharist. Next, “Amen” to the priesthood which confects this Eucharist and the authority of the bishop who ordained the priest and the Pope that holds them in full communion with the See of Peter. Finally, “Amen” to all that the Church proposes as being true and definitively taught as worthy of our belief. So, in order to make a genuine communion, a person receiving the Eucharist must be in full communion with the Church — that is, he or she accepts everything that the Church teaches. To believe in anything less makes that person’s “Amen” a disingenuous act. A true “Amen” links us to Jesus and nourishes us into everlasting life.
This is precisely the reason why Catholics don’t offer holy Communion to non-Catholics. The reasoning is really quite simple. If, for example, a Protestant or a Jew was to come up in the Communion line and the priest would say, “The Body of Christ,” the only response would be “Amen.” However, since neither Protestants nor Jews believe in the Eucharist in the same way that Catholics do, the priest would be asking the Protestant or Jew to violate their conscience in saying “Amen” to realities they do not accept.
(Sources: Catholic Answers, Catholic Exchange)
Read more in the “Why Do We Do That?” series from Deacon Mike Fritz.