Cyclopedia of Biblical, Theological and Ecclesiastical Literature
(Hebrew amen', אָמֵן , ἀμήν ), a particle of attestation adopted into all the languages of Christendom.
I. This word is strictly an adjective, signifying "firm," and, metaphorically, "faithful." Thus, in Revelation 3:14, our Lord is called "the amen, the faithful and true witness." In Isaiah 65:16, the Hebrew has "the God of amen," which our version renders "the God of truth," i e. of fidelity. In its adverbial sense amen means certainly, truly, surely. It is used in the beginning of a sentence- by way of emphasis — rarely in the Old Test. (Jeremiah 28:6), but often by our Savior in the New, where it is commonly translated "verily." In John's Gospel alone it is often used by him in this way double, i.e. "verily, verily." In the end of a sentence it often occurs singly or repeated, especially at the end of hymns or prayers, as "amen and amen" (Psalm 41:14; Psalms 72:19; Psa 89:53). The proper signification of it in this position is to confirm the words which have preceded, and invoke the fulfillment of them: "so be it,! fiat; Sept. γένοιτο . Hence in oaths, after the priest has repeated the words of the covenant or imprecation, all those who pronounce the amen bind themselves by the oath (Numbers 5:22; Deuteronomy 27:15; Deuteronomy 27:17 : Nehemiah 5:13; Nehemiah 8:6; 1 Chronicles 16:36; comp. Psalms 106:48). (See OATH).
II. In the public worship of the primitive churches it was customary for the assembly at large to say Amen at the close of the prayer; a custom derived from apostolic times (1 Corinthians 14:16). Several of the fathers refer to it. Jerome says that in his time, at the conclusion of public prayer, the united voice of the people sounded like the fall of water or the noise of thunder. Great importance was attached to the use of this word at the celebration of the eucharist. At the delivery of the bread the bishop or presbyter, according to the Apostolical Constitutions, is directed to say, "The body of Christ;" at the giving of the cup the deacon is instructed to say, "The blood of Christ, the cup of life;" the communicant is directed on each occasion to say "Amen."' This answer was universally given in the early Church. (See RESPONSE).
III. It is used as an emphatic affirmation, in the Sense "so be it," at the end of all the prayers of the Church of England. It is sometimes said in token of undoubting assent, as at the end of the creed, Amen, "So I believe." The order of the Church of England directs that the people shall, at the end of all prayers, answer Amen." — Bingham, bk. 15, ch. 3, § 25.
Special treatises on the subject are Kleinschmidt, De particula Amen (Rint. 1696); Weber, De voce Amen. (Jen. 1734); Wernsdorf, De Amen, liturgico (Viteb. 1779); Brunner, De voce Amen (Helmst. 1678); Fogelmark, Potestas verbi אָמֵן (Upsal. 1761); Meier, Horoe philol. in Amen (Viteb. 1687); Treffentlich, De אָמֵן (Lips. 1700); Vejel, De vocula Amen (Argent. 1681); Bechler, Horoe philol. in Amen (Wittemb. 1687).
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McClintock, John. Strong, James. Entry for 'Amen'. Cyclopedia of Biblical, Theological and Ecclesiastical Literature. https://www.studylight.org/encyclopedias/eng/tce/a/amen.html. Harper & Brothers. New York. 1870.