Amen is a Hebrew adjective that originally meant “reliable, sure, true.” When it is used as an adjectival verb it meant, “It is reliable or true.” Another Hebrew verb, aman means to “support, sustain.”
When this word was used alone it was used to indicate a meaning of “surely!” or “in the very truth!” For example, when it was used after the phrase “Blessed be Yahweh forever” it means “Yes indeed!” or “May it be so in very truth.” (Psalms 41:13; 72:19, 89:52; 106:48, I Chronicles 16:36, Nehemiah 8:6)
When amen is used in the New Testament it usually is used as a liturgical formula at the end of a doxology. It seems from I Corinthians 14:16 that in the early church the congregation was expected to respond with a spoken “amen.”
Those who pray or listen as another prays and at the end says, amen, put themselves into the statement of what was said or asked with all earnestness of faith and intensity of desire. Our word is often used to show strong agreement and/or sympathy with the one who is speaking or asking.
Jesus seems to have given this word a slightly different meaning when he uses it in the Gospels. When Jesus used this word, He used it to mean “surely” or “truly.” All four Gospels (Matthew, Mark, Luke, and John) show this word to be an introductory formula. For example:
“amen lego hymin” “truly, I say to you” which serves to emphasize the authority of what He is about to say. This phraseology is peculiar to Jesus in the New Testament and probably reflects His divine self-consciousness. Jesus does not have to wait until after he has spoken to ratify what is said; all that Jesus said had the mark of absolute and certain truth.
Bibliography. Walter A. Elwell, Evangelical Dictionary of Theology, 2nd Ed, p. 51
The International Standard Bible Encyclopedia, ed. G. W. Bromiley, p. 110