In Maseches Shabbos (119b) it says: “Rabi Yehoshua ben Levi says: ‘Anyone who answers amen yehei Shemei Rabba Mevarach with all his strength has his decree torn up.’”
The holy Zohar promises the tearing up of a decree for anyone who answers amen, and not necessarily only one who answers “amen yehei Shemei rabbah, as it says (Tikkunei Zohar 40 1): “One who answers amen with all his might has a decree of seventy years torn up” (see Shomer Emunim, Ma’amar Pischu She’arim 1).
The Birchos Shamayim wrote that Dovid Hamalech thanked Hashem for the gift of amen in Tehillim (106:48): “Baruch Hashem Elokei Yisrael min ha’olam v’ad ha’olam v’amar kol ha’am amen halleluk-ah.” And the passuk is explained: We have to bless and thank Hashem for this wonderful gift of “v’amar kol ha’am amen” in whose power the decree is lifted from Am Yisrael, and from the fact that Hashem gave us this gift, it is evident that He is the Elokei Yisrael, Who seeks the benefit of them to protect them and save them from all bad (Tal Hashamayim Shabbos Shuvah).
