שְׁמַ֖ע יִשְׂרָאֵ֑ל יְהוָ֥ה אֱלֹהֵ֖ינוּ יְהוָ֥ה׀ אֶחָֽד
Hear, o Israel: Yahweh is our God; Yahweh alone (my translation)
The Shema is often cited as evidence of strict monotheism. However, most modern biblical scholars agree that the Shema is not about the “number” of God, but instead, is about how Yahweh is the only God with whom Israel is to have a covenantal relationship with. A parallel would be Deut 5:7, a rendition of the Decalogue:
Thou shalt have no other Gods before me. (cf. Exo 20:3 [exegeted here])
According to biblical scholars such as Michael Coogan, this commandment and the Shema implicitly recognises the ontological existence of other gods (cf. Gen 20:13). As in a marriage, one of the primary analogues for the covenant, Israel was to be faithful, like a wife to her husband. When the prophets condemn the Israelites for having worshipped other gods in violation of this commandment, the metaphors of marital and political fidelity are often invoked, sometimes graphically (e.g., Ezek 16:23-24; 23:2-12; Jer 2:23-25; 3:1-10). Yahweh is a jealous husband (e.g., Exo 34:14) and the worship of other gods, or making alliances with foreign powers, provokes his rage (Michael D. Coogan, The Old Testament: A historical and literary introduction to the Hebrew Scriptures [New York: Oxford University Press, 2006], 176, 116).
Additionally, there has been a lot of linguistic nonsense about the Hebrew numeral אֶחָד which simply means one (not “plural one” or some other nonsense one finds among some Trinitarians). See my blog post on Gen 2:24 addressing this pathetic argument, or the work of Anthony F. Buzzard, who has done a lot of great work on this particular issue (while I disagree with Buzzard’s Socinian Christology, he shows the Trinitarian arguments are utter nonsense).
For those wishing to delve further into the issue of "monotheism" in the book of Deuteronomy, I would highly recommend Nathan MacDonald, Deuteronomy and the Meaning of "Monotheism" (2d ed.: Mohr Siebeck, 2012).