Amen is very likely the most widely uttered religious word on earth, and one of a tiny number that passed from Hebrew into Greek, Latin, Arabic and hundreds of vernaculars without being translated. In the Hebrew Bible it is an assent, from a root meaning to be firm, reliable, true, spoken to affirm an oath, a blessing or a curse and to make it one's own; to say amen to a blessing is to join in it and, the rabbis held, to gain a share in it.
From the synagogue it entered Christian worship as the fixed response to prayer, carried untranslated because it was already felt to be sacred sound, and it was similarly received into Islamic usage, said after the recitation of the opening sura in prayer. The word's refusal to be translated is itself instructive: across three related traditions that disagree about almost everything else, the same short Hebrew affirmation seals the prayers of all of them. Its meaning has never been mysterious, it says, in effect, so it is, or so may it be, but its function, to make a communal act one's own by voicing agreement, is remarkably constant wherever it travelled.