The Shema is both a prayer and a statement of the most fundamental aspect of Jewish theology: God’s oneness. The words of the first line of the Shema are found in Deuteronomy 6:4 (“Hear O Israel, the Lord is our God, the Lord is one”) followed by the rest of the first paragraph Deuteronomy 6:5-9 (known as V’ahavta). Two more biblical selections from Deuteronomy 11:13-21 (V’hayah) and Numbers 15:37-41 (Va’yomer) complete the Shema. In every siddur, however, one will find a small line of print that follows the primary line of the Shema. This line, Baruch shem k’vod malchuto l’olam va’ed (‘Blessed be the name of His glorious kingdom for ever and ever) is almost always recited in a whisper.* The Talmud explains why:
According to Rabbi Shimon ben Lakish,, the patriarch Jacob, some time before his demise, wished to “reveal to his sons the ‘end of days.’” [The Almighty, however, did not agree.] Whereupon the Divine presence departed from him. Said [Jacob], ‘Perhaps, God forbid, there is one unfit among my children…’ [But] his sons answered him, ‘Hear O Israel [Israel is another name for their father Jacob], the Lord our God the Lord is One (the Shema): Just as there is only One in your heart, so is there in our heart only One.’ At that moment, our father Jacob opened [his mouth] and exclaimed, ‘Blessed be the name of His glorious kingdom for ever and ever” (Pesachim 56a).
Because Jacob connected this verse to the recitation of Shema, the rabbis felt that it should be included in the formal recitation of the prayer. However, uncomfortable with intruding on biblical verses but also with disregarding the declaration of Jacob, the sages “enacted that it should be recited quietly”* (ibid)
*With the exception of Yom Kippur, when it is recited out loud.