a-men' (in ritual speech and in singing a-men', a'men) ('amen; amen, = "truly," "verily"): Is derived from the reflexive form of a verb meaning "to be firm," or "to prop." It occurs twice as a noun in Isa 65:16, where we have (the King James Version, the Revised Version (British and American)) "God of truth." This rendering implies the pointing 'omen or 'emun i.e. "truth," or "faithfulness," a reading actually suggested by Cheyne and adopted by others. "Amen" is generally used as an adverb of assent or confirmation--fiat, "so let it be." In Jer 28:6 the prophet endorses with it the words of Hananiah. Amen is employed when an individual or the whole nation confirms a covenant or oath recited in their presence (Nu 5:22; De 27:15 ff; Ne 5:13, etc.). It also occurs at the close of a psalm or book of psalms, or of a prayer.
That "Amen" was appended to the doxology in the early church is evident both from Paul and Rev, and here again it took the form of a response by the hearers. The ritual of the installation of the Lamb (Re 5:6-14) concludes with the Amen of the four beasts, and the four and twenty elders. It is also spoken after "Yea: I come quickly" (Re 22:20). And that Revelation reflects the practice of the church on earth, and not merely of an ideal, ascended community in heaven, may be concluded from 1Co 14:16, whence we gather that the lay brethren were expected to say "Amen" to the address. (See Weizsacker's The Apostolic Age of the Christian Church, English translation,II , 289.)