Musar for Shabbat 238:1

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.

And Chazal have said (Shabbath 119b): "The breath of the mouth that has sinned is not like the breath of the mouth that has not sinned." ["the breath of the mouth that has sinned":] That is, he who has sinned only sometimes, by chance, as we have explained in Chapter VII. How much more so if the breath of his mouth is habituated to sin every day and every hour. And all this, even if his mouth sins in other forbidden things; how much more so, in the issur of lashon hara and rechiluth by which he certainly undermines the prayer that he prays after this; and it does not ascend unless he undertakes to repent of this, as we explained above in Chapter VII in the name of the holy Zohar. And we can say that this is the intent of the verse in Koheleth 7:13: "See the act of G-d. For who can straighten out what he has made crooked?" That is, set your eyes and your heart on the G-dly things, the holy sanctuaries and the celestial lights created by you on high by your Torah and your mitzvoth — that they be created in perfection, by all the preparations needed for this and also by "clean tools," as explained above. For who can straighten out what he has made crooked? This is, if you damage them, G-d forbid, they will remain forever defective and damaged, and this will cause you eternal worry and constant grief in remembering that it was your slothfulness that caused all this. For they [(the heavenly edifices)] are not like the buildings of this world, which, if one damages them, another builder can repair them. Not so, with the desired [eternal] end. As stated by the tanna in Avoth 1:13: "If I am not for myself, who is for me?" And this is the intent of the verse "For who can straighten out what he has made crooked?"