literally "true" and used as a substantive, "that which is true," "truth," (Isaiah 65:16) a word used in strong asseverations, fixing, as it were, the stamp of truth upon the assertion which it accompanied, and making it binding as an oath. Comp. (Numbers 5:22) In the synagogues and private houses it was customary for the people or members of the family who were present to say "amen" to the prayers which were offered. (Matthew 6:13; 1 Corinthians 14:16) And not only public prayers, but those offered in private, and doxologies, were appropriately concluded with "amen." (Romans 9:5; 11:36; 15:33; 16:27; 2 Corinthians 13:14) etc.
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Smith, William, Dr. "Entry for 'Amen,'". "Smith's Bible Dictionary".