(ג ה) "עבדים מבית מצרים מארץ הוצאתיך אשר אלקיך ‘ה אנכי"
This passuk is the source for the mitzvah of emunah that obligates a person to believe that there is a Creator and Leader in the world (Rambam, Sefer Hamitzvos Mitzvas Aseh 1; Sefer Hachinuch 25). In contrast to most of the mitzvos that are bound by time, place or other elements, this mitzvah is constant, and whenever a person thinks about the Metzius Hashem, he fulfills it. The Ohr Zarua (1 140) writes that this is why Chazal did not institute to make a brachah before it, as one makes a brachah on the other mitzvos, because they only instituted to say a brachah before a mitzvah that comes from time to time. This is because the brachah on the mitzvah expresses the love for the mitzvah at a time when a person has an opportunity to fulfill it, which belongs only to mitzvos whose obligation is renewed from time to time, and not mitzvos that have to be fulfilled at all times, and there is never something new about them.
Although there is no special brachah for this mitzvah, the mitzvos of answering amen after brachos and tefillos were instituted because of this mitzvah of emunah. As Rabbeinu Bechayei says (Shemos 14:1): "And because emunah is the fundamental of the entire Torah, Chazal instituted that we answer amen in tefillah and in brachos, as it is derived from the term ‘emunah’ and ‘hoda’ah,’ that one accepts upon himself the words of the mevarech and admits (is modeh) to them." The Megaleh Amukos wrote (Parashas Ha’azinu): "The ikkar of emunah is contingent on answering amen."
Indeed, the meaning of answering amen is the declaration of absolute emunah in the Creator. The Gemara asks in Maseches Shabbos (119b), "What is amen?" And it replies that the word amen is an acronym for the words .א-ל מלך נאמן Rashi explains (ibid ad loc. K-l) that one who answers amen testifies of his Creator that He is a ‘K-l Melech Ne’eman.’ The Tosafos (ibid ad loc. Amar Rabi Chanina) derives from these words of the Gemara that when answering amen, one should have in mind that Hashem is the "K-l Melech Ne’eman."
