Interestingly, Amen makes its first Biblical appearance twice in one verse. The verse in question involves a unique ceremony, performed in front of a priest, to determine if a married woman was unfaithful or not to her husband. If she was not an adulteress then the water given her to drink would be harmless. If she was guilty of such a sin, however, the water was meant to cause the following penalty.
And this water that causeth the curse shall go into thy bowels, to make thy belly to swell, and thy thigh to rot: And the woman shall say, Amen, Amen (Numbers 5:22).
New Testament usage
In the New Testament, the Greek word translated Amen (Strong's #G281) means the same thing as its Old Testament counterpart. Many times, in the KJV, it is translated as "verily" (Matthew 5:18, 26, etc.), a word commonly spoken by Jesus. Its first occurrence is found at the end of what is called the Lord's Prayer (Matthew 6:13).
The word is recorded 51 times in 50 New Testament verses, with Revelation 7:12 being the only one to list it twice. The book of Revelation contains the most uses of the word with 10. Perhaps not surprisingly, it is the final word of the last Bible book.
The grace of our Lord Jesus Christ be with you all. Amen (Revelation 22:21, KJV).
The Apostle Paul used the word Amen in all fourteen of his writings. Additionally, all four gospel writers end their books with the word (Matthew 28:20, Mark 16:20, Luke 24:53, John 21:25). The only New Testament writings that lack it are James and 3John.
A unique word
The special place of Amen in religion and society is highlighted by the 1906 Jewish Encyclopdia. In its article on the term it states it, "is perhaps the most widely known word in human speech; being familiar to Jews, Christians, and Mohammedans (Islam)."
The 1911 Encyclopedia Britannica also noted a few facts about the use of Amen in the world.
"Among certain Gnostic sects Amen became the name of an angel, and in post-biblical Jewish works exaggerated statements are multiplied as to the right method and the bliss of pronouncing it. It is still used in the service of the synagogue, and the Mahommedans (Islam) not only add it after reciting the first Sura of the Koran, but also when writing letters . . . and repeat it three times . . . as a kind of talisman."