The Jewish Study Bible on the Shema

Many modern readers regard the Shema as an assertion of monotheism, a view that is anachronistic. In the context of ancient Israelite religion, it served as a public proclamation of exclusive loyalty to YHWH as the sole Lord of Israel . . . the v. makes not a quantitative argument (about the number of deities) but a qualitative one, about the nature of the relationship between God and Israel. Almost certainly, the original force of the v., as the medieval Jewish exegetes [noted], was to demand that Israel show exclusive loyalty to our God, YHWH--but not thereby to deny the existence of other gods. In this way, it assumes the same perspective as the first commandment of the Decalogue, which, by prohibiting the worship of other gods, presupposes their existence. (The Jewish Study Bible [2d ed.; New York: Oxford University Press, 2014], 361)