Certainly, Christ made God's many promises come true.
For that reason, because of our message,
people also honor God by saying, "Amen!"
[See] 2 CORINTHIANS 1:20
THE LITTLE WORD AMEN indicates strong affirmation and means "let it be so." It expresses the faith we should have when we pray.
Christ said, "Have faith that you will receive whatever you ask for in prayer" (Matthew 21:22). He also said, "That's why I tell you to have faith that you have already received whatever you pray for, and it will be yours" (Mark 11:24). The Samaritan woman received what she asked for because she didn't stop asking and firmly believed. In response, the Lord said to her, "Woman, you have strong faith! What you wanted will be done for you" (Matthew 15:28). James also said, "When you ask for something, don't have any doubts. A person who has doubts is like a wave that is blown by the wind and tossed by the sea" (James 1:6).
So as the author of Ecclesiastes said, "The end of something is better than its beginning" (Ecclesiastes 7:8). For at the end of your prayers, you say Amen with heartfelt confidence and faith. When you say Amen, the prayer is sealed, and it will be certainly heard. Without this ending, neither the beginning nor the middle of the prayer will be of any benefit."
Martin Luther, By Faith Alone, World Bible Publishers, Inc., entry for June 2 (copyright 1998). This is presented here for religious and educational purposes only. This one page of text out of a book of over 365 pages is used in reliance on the Fair Use Doctrine of the United States Code, 17 USC 107. Martin Luther was born in 1483 and began the reformation. [For more you can see Catholic Encyclopedia on Luther, however this was published in 1913 and is more critical of Luther than is true today. For example, see the 1999 Joint Declaration on Justification by the Catholic and Lutheran Churches. See also The Catechism of the Catholic Church, section 838 and 818 which says: "All who have been justified by faith in Baptism are incorporated into Christ; they therefore have a right to be called Christians, and with good reason are accepted as brothers in the Lord by the children of the Catholic Church."]
American Heritage Dictionary, Amen: "INTERJECTION: Used at the end of a prayer or a statement to express assent or approval."
The contemporary catechism sees the Amen as an affirmation of faith; and in regards to the Lord's Prayer, it says: "Then, after the prayer is over you say 'Amen,' which means 'So be it,' thus ratifying with our 'Amen' what is contained in the prayer that God has taught us." (Quoting St. Cyril of Jerusalem.) Section 2856.
The Council of Trent indicates: "Its meaning may be said to be: Know that thy prayers are heard." The Council goes on to report these other ideas about its meaning: "The Septuagint interprets it, 'So be it'; others translate it, 'Verily': Aquila renders it, 'Faithfully'. Which of these versions we adopt, is a matter of little importance, provided we understand the word to have the sense already mentioned namely...it signifies the concession of what has been prayed for."