… And all the people shall say, ‘Amen!’ (Deuteronomy 27:26)
Citing the above verse the Gemara states that by reciting Amen the Jewish people obligated themselves to abide by the Torah. In addition, reciting Amen to a prayer is an expression of hope that the prayer will be fulfilled (Shevuot 36a).
The Shulchan Aruch (Code of Jewish Law, written in 1563) rules that if someone hears another person recite a blessing, they are obligated to respond ‘Amen’ (Orach Chaim 215:2). Rabbi Yisrael Meir HaKohen Kagen (known as the Chofez Chaim, d. 1933) rules that this applies even if one only heard God’s Name and the conclusion of the blessing (Mishnah Berurah ibid. 6). One should respond Amen immediately after the blessing is completed; it should not be before completion or a long time after.
It is necessary to respond Amen when a child makes a blessing, provided that the child made the blessing over something in which they are actually obligated. If the child was merely practising the blessing to learn it, such as a Bar Mitzvah student rehearsing the blessings for his Torah or Haftarah reading, one should not respond Amen (Orach Chaim215:3). Similarly, if an adult errs and makes an unnecessary blessing one should also not respond Amen.
The Gemara cites the verse ‘Declare the greatness of the Lord with me, and let us exalt His name together’ (Psalms 34:4) to mean that those responding ‘Amen’ should not make their voice louder or quieter than the person who recited the blessing (Brachot 45a).
The root of the word (אמן) is the same as the word emunah (אמנה) meaning faithfulness or belief. The Gemara also teaches that Amen is an acronym for א-ל מלך נאמן – God, trustworthy King (Shabbat 119b). The Shulchan Aruch requires those responding Amen to have in mind that they are testifying to the truth of the blessing and believing that God will bring about the prayer in the future (ibid. 124:6).
Reciting Amen transforms a personal blessing into a communal prayer. At the same time it affords others the opportunity to acknowledge God and demonstrate their faith in Him.