The American Church Dictionary and Cycopedia
A Hebrew word meaning "so be it," or "so it is," as it is used at the end of prayers, hymns or Creed. It signifies approval of, or assent to, what has gone before. The use of the "Amen" in Public Worship emphasizes the Priesthood of the Laity, as for example, in the consecration of the elements in the Holy Communion, while the celebrating Priest stands before God offering to Him this holy Oblation, he does it in company with all the faithful, and to signify their cooperation with him in this great act they say "Amen," adopting his words and acts as their own. In the early Church the "Amen" was said with such heartiness, an ancient writer describes it as sounding "like a clap of thunder." (See RESPONSIVE SERVICE).
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Miller, William James. Entry for 'Amen'. The American Church Dictionary and Cyclopedia. https://www.studylight.org/dictionaries/eng/acd/a/amen.html. 1901.