Musar על שבת 238:5

9) When one prays with the congregation, he has [the opportunity of saying] Barchu, Kedushah, and Amen yehei shemei rabbah, each of which, in itself, is awesome. For through "Barchu" we make a crown for the Holy One Blessed be He, as we find in Midrash Konein. And through Kedushah we fulfill (Vayikra 22:33): "And I shall be sanctified in the midst of the children of Israel." And through this, holiness is conferred upon us, as the verse concludes: "I am the L-rd who sanctifies you." And through "Amen, yehei shemei rabbah" one is forgiven for all of his sins, as our sages of blessed memory have said (Shabbath 119b): "If one says 'Amen, yehei shemei rabbah' with all of his strength [i.e., with all of his intent], even if there is a trace of heresy in him, he is forgiven."

And now, if we come to the count, let a man reflect upon the great zeal he should devote to this; that is, to strengthen himself and to pray with a minyan each day. For from one day alone, from the prayers of shacharith and minchah, he has thirty-eight Amens from nineteen blessings of prayer [i.e., the Amidah], respectively [(And our sages of blessed memory have said (Shabbath 119b): "If one is careful in the answering of Amen, the gates of Gan Eden are opened for him, as it is written (Isaiah 26:2): 'Open, you gates, and let there enter a righteous nation, keeper of emunim [faith].' Read it not "emunim," but "Amenim."'")], and eight times "Amen, yehei shemei rabbah," and another sixteen Amenim from the Kadishim and two Kedushoth and Barchu.