The Rabbis of the Talmud teaches that “Amen” is an acrostic from the first letters of the three words, El Melech Ne’eman, meaning the Lord is trustworthy. “Amen” appears the first time in Numbers 5:22. The Lexicon states that it is an adverb of confirmation meaning truly, very, certainly, and so be it.
“If you declare with your mouth, “Jesus is Lord,” and believe in your heart that God raised him from the dead, you will be saved. For it is with your heart that you believe and are justified, and it is with your mouth that you profess your faith and are saved.”