Easton's Bible Dictionary (1897)/Amen
Amen, this Hebrew word means firm, and hence also faithful (Revelation 3:14). In Isaiah 65:16, the Authorized Version has "the God of truth," which in Hebrew is "the God of Amen." It is frequently used by our Saviour to give emphasis to his words, where it is translated "verily." Sometimes, only, however, in John's Gospel, it is repeated, "Verily, verily." It is used as an epithet of the Lord Jesus Christ (Revelation 3:14).
It is found singly and sometimes doubly at the end of prayers (Psalms 41:13; 72:19; 89:52), to confirm the words and invoke the fulfilment of them. It is used in token of being bound by an oath (Numbers 5:22; Deuteronomy 27:15-26; Nehemiah 5:13; 8:6; 1 Chronicles 16:36). In the primitive churches it was common for the general audience to say "Amen" at the close of the prayer (1 Corinthians 14:16).
The promises of God are Amen; i.e., they are all true and sure (2 Corinthians 1:20).