Fausset's Bible Dictionary
("firm", "faithful", else "verily".) Jesus is "the Amen, the, faithful and true witness" (Revelation 3:14). Compare 2 Corinthians 1:20; John 1:14; John 1:17; John 14:6. "The God of Amen" (Hebrew for "truth") (Isaiah 65:16). Jesus alone introduces His authoritative declarations with Amen in the beginning; in Matthew, Mark, and Luke, singly, in John (John 3:3; John 3:5; John 3:11; John 10:1) always doubled. It is most marked how the apostles and others avoid the use of it in the beginning, which is His divine prerogative. Jeremiah 28:6 is not an exception; it is praying for the divine ratification of what preceded. In oaths those who pronounce the "Amen" bind themselves by the oath (Numbers 5:22; Deuteronomy 27:15-26).
God alone can seal all His declarations of promise or threat with the "Amen," verily, in its fullest sense; our assertions mostly need some qualification. As John records Christ's discourses on the deeper things of God, which man is slow to believe, the double Amen is appropriately found at the beginning of such discourses 25 times. Amen was the proper response to a prayer, an oath, or a solemn promise (1 Kings 1:30; Nehemiah 5:13; Nehemiah 8:6; 1 Chronicles 16:36; Jeremiah 11:5); the God of Amen witnesses our covenants. Jewish tradition states that the people responded to the priest's prayer not "Amen," but, "Blessed be the name of the glory of His kingdom for ever." But in synagogues, as in the Christian assemblies, and in family and private prayers, Amen was the response (Matthew 6:13; 1 Corinthians 14:16).
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Fausset, Andrew R. Entry for 'Amen'. Fausset's Bible Dictionary. https://www.studylight.org/dictionaries/eng/fbd/a/amen.html. 1949.