Listen, Israel: the LORD is our God, the LORD alone. (Deuteronomy 6:4)
What, precisely, did the rabbis think happened when one recites the Shma? We find an answer in the reply of the Tannaitic master Rabbi Joshua ben Korhah to the question of why Deut 6:4-9 is positioned before 11:13-21:
“so that one might accept upon himself the yoke of the kingdom of heaven first; afterwards, he accepts upon himself the yoke of the commandments.”
…Rabbi Joshua sees the Shma, therefore, as the acclamation of God’s kingship. Only in light of such an acclamation do the mitsvot make sense…
To respond wholeheartedly to that demand, to accept the yoke of the kingdom of heaven, is to make a radical change, a change at the roots of one’s being. To undertake to live according to Halakhah is not a question of merely raising one’s moral aspirations or affirming “Jewish values,” whatever that means. To recite the Shma and mean it is to enter into a supramundane sovereignty, to become a citizen of the kingdom of God.
(J. D. Levenson, ‘Sinai and Zion’)