The Midrash (Devarim Rabbah 7 1) says: "Rabbi Yuden said anyone who answers amen in Olam Hazeh merits to answer amen l’asid lavo." Rav Eliyahu Hakohein Ha’itamari explained that the words of Rabbi Yuden "Kol ha’oneh amen" are mechadesh that anyone who answers amen in this world, even once in his lifetime, even if he mistakenly did not answer it according to halachah, merits to answer amen in Olam Haba. How much more so is this the case for one who answers amen according to halachah at every opportunity – and his reward is surely endless. (Chut Shel Chessed, Ki Savo)
Someone Whose Purpose Is Answering Amen
Rav Eliyahu further explained the words of the Midrash: The Ari Hakadosh revealed (Eitz Hada’as Tov, Ki Seitzei) that when a person feels a bond and a special warmth towards a certain mitzvah, he should conclude that being strict with this mitzvah is his purpose in the world. Therefore, it behooves him to think constantly and be extra careful about fulfilling it. This is what this Midrash alludes to: "Anyone who answers amen," meaning, when someone who cleaves to the mitzvah of answering amen, it is a sign that this is his purpose in "Olam Hazeh." As such, when his time comes, he will merit "to answer amen l’asid lavo," and he will not have to come down to this world again, because he carried out his mission faithfully, as was expected of him (ibid).
Mordechai Taught Esther to Answer Amen
The Rema rules (Orach Chaim 124 7) that "he should teach his young children to answer amen, because as soon as a baby answers amen he has a share in Olam Haba." His words are based on the words of the Gemara in Sanhedrin (110b): "From when does a child earn Olam Haba...It is taught in the name of Rabi Meir: From when he answers amen." Based on this, Rav Eliyahu explained the passuk about Mordechai (Esther 2:7): "Vayehi omen es Hadassah he Esther bas dodo ki ein lah av v’eim." Because Esther did not have a father and mother to teach her to answer amen, therefore, "Vayehi omen," Mordechai made sure to be mechanech her from an early age to answer amen (Minchas Eliyahu Ch. 32).
Answering Amen Sanctifies the Person
When a person answers amen, he acquires a holiness for his soul, and with this, his soul can return to his body during techiyas hameisim. For this reason, Rabi Meir says (Sanhedrin 110b) that "From when does a child come to Olam Haba?... From the time he says amen" (Semuchim La’ad, Naso; Aggadas Eliyahu, Shevi’is Ch. 4).
