Before Israel sinned, what is written? "And the appearance of the glory of the LORD was like a devouring fire." Rabbi Abba bar Kahana said: Seven walls of fire were glittering, one within the other, and Israel would look upon them and not be afraid or terrified. But once they sinned, they could not even look upon the face of the intermediary [Moses], as it is written, "And Aaron and all the children of Israel saw the face of Moses... and they were afraid to come near him." Rabbi Pinchas in the name of Rabbi Chanina said: Even the intermediary sensed the sin, as it is said, "Kings of armies flee." Rabbi Yudan in the name of Rabbi Eivo said: It is not written here "angels of armies," but rather "kings of armies" - the kings of the angels. Even Michael and Gabriel could not gaze upon his face. Once Israel sinned, he could not even gaze upon the face of the recruits [the lowest ranks], as it is said (Deuteronomy 9:19), "For I was afraid of the anger and the wrath."
Seven Walls of Fire That Israel Once Faced Unafraid
Yalkut Shimoni on Torah 362:9
עַד שֶׁלֹּא חָטְאוּ יִשְׂרָאֵל מַה כְּתִיב וּמַרְאֵה כְּבוֹד ה' כְּאֵשׁ אֹכֶלֶת. אֲמַר רַבִּי אַבָּא בַּר כַּהֲנָא. שֶׁבַע מְחִיצוֹת שֶׁל אֵשׁ הָיוּ נוֹצְצוֹת זוֹ בְּזוֹ, וְהָיוּ יִשְׂרָאֵל רוֹאִין וְלֹא מִתְיָרְאִין וְלֹא מִתְפַּחֲדִין, וְכֵיוַן שֶׁחָטְאוּ אֲפִלּוּ פְנֵי הַסַּרְסוּר לֹא הָיוּ יְכוֹלִין לִרְאוֹת, הָדָא הוּא דִּכְתִיב (שם) "וַיַּרְא אַהֲרֹן וְכָל בְּנֵי יִשְׂרָאֵל אֶת (פְּנֵי) מֹשֶׁה וְגוֹ' וַיִּירְאוּ מִגֶּשֶׁת אֵלָיו". רַבִּי פִּנְחָס בְּשֵׁם רַבִּי חֲנִינָא. אַף הַסַּרְסוּר הִרְגִּישׁ בָּעֲבֵרָה, שֶׁנֶּאֱמַר "מַלְכֵי צְבָאוֹת יִדֹּדוּן". רַבִּי יוּדָן בְּשֵׁם רַבִּי אֵיבוֹ. "מַלְאֲכֵי צְבָאוֹת" אֵין כְּתִיב כָּאן, אֶלָּא "מַלְכֵי צְבָאוֹת", מַלְכֵיהֶן דְּמַלְאָכִים, אֲפִלּוּ מִיכָאֵל וְגַבְרִיאֵל לֹא הָיוּ יְכוֹלִין לְהִסְתַּכֵּל בְּפָנָיו שֶׁלּוֹ, כֵּיוָן שֶׁחָטְאוּ יִשְׂרָאֵל אֲפִלּוּ פְּנֵי הַגּוּלַיְירִין לֹא הָיָה יָכוֹל לְהִסְתַּכֵּל, שֶׁנֶּאֱמַר (דברים ט, יט) "כִּי יָגֹרְתִּי מִפְּנֵי הָאַף וְהַחֵמָה".