Why do people say “amen” at the end of a prayer?

 
The word “amen,” in both English and Greek, is a transliteration of the Hebrew ʼa‧men′. The expression, usually uttered in unison by listeners to a prayer, oath, blessing, or curse, basically means “so be it,” or “surely.” Saying it serves to indicate agreement with the sentiments just expressed. According to one reference work, “the word connotes certainty, truthfulness, faithfulness, and absence of doubt.” In Bible times, the expression also legally obliged its user with regard to an oath or covenant and its consequences.—Deuteronomy 27:15-26.
 
In his preaching and teaching, Jesus introduced some of his statements with the word “amen.” By so doing, he underlined the absolute reliability of what he was about to say. In these cases, the Greek word a‧men′ is translated “truly” or “verily.” (Matthew 5:18; 6:2, 5; King James Version) When doubled, as is the case throughout the Gospel of John, Jesus’ expression is translated “most truly.” (John 1:51) Jesus’ use of amen in this way is said to be unique in sacred literature.
 
In the Christian Greek Scriptures, the title “Amen” is applied to Jesus to indicate that his witness is “faithful and true.”—Revelation 3:14.