On Yom Kippur, why do we have the custom of saying the phrase recited after the “Shema” out loud when we usually recite it in a whisper? Or why do we usually say it in a whisper if on Yom Kippur we say it out loud?
When reciting the “Shema” throughout the year we say the first verse, “Hearken Israel, the L-rd is our G‑d, the L-rd is One” out loud. After that we say in a whisper, “Baruch shem kevod malchuto l’olam va’ed” – “Blessed be the Name of His glorious majesty forever”. It is this second phrase that we recite out loud on Yom Kippur.
This is because it is a song of praise sung by the angels to G‑d, and during our more mundane plane of existence throughout the year we may only whisper it with great trepidation. However, on Yom Kippur, when through repentance and fasting we gain a more singular devotion to G‑d as that of the angels, we are able to recite this praise more fully, i.e. out loud.
The Midrash (Deut. Raba 2:36) records: When Moses ascended on high to receive the Torah, he heard the ministering angles say before G‑d, “Blessed be the Name of His Glorious Majesty Forever”, and he brought this verse of praise down from Heaven for Israel. Why then does Israel not pronounce it openly? Rabbi Assi said: This may be likened to one who took an ornament from the king’s palace. He gave it to his wife but said to her, “Do not wear it publicly but only in the house”. So Israel received this very special formula of prayer from the highest spiritual realm, and therefore only uses it in this world discreetly. On Yom Kippur however, when Jews are as pure and elevated as the ministering angels, they rightfully openly pronounce this song of the angels.