Chazal prove the obligation to answer amen with the passuk (Devarim 32:3): “Ki Shem Hashem ekra – havu godel l’Elokeinu,” and explain that Moshe Rabbeinu said to Am Yisrael: “While I mention the Name of HaKadosh Baruch Hu in the brachah – you should bring Him greatness by answering amen” (Rashi Brachos 21a, based on Yoma 37a). From the fact that Chazal attribute the obligation to answer amen to the passuk “Havu godel l’Elokeinu,” the Kli Yakar learns that aside for having to think while answering amen about the Sheimos of Havay-ah and Adnus, which are numerically equivalent to amen, we should also have in mind the Name of Elokim, because that is also equivalent to amen – as follows: The name אלקים is numerically equivalent to 86, and when we add the five letters of the word, then we reach 91, which is the value of amen. (Sifsei Da’as Ha’azinu 370)
