The common idea that Judaism was always purely monotheistic and that Christianity later departed from this is historically problematic. The term "monotheism" is modern, and historical views of God were more dynamic, moving between "monotheism" and "polytheism" or including various configurations. In ancient Israel and Second Temple Judaism, God (YHWH) had to assert himself against other deities like El and Ba'al. Sometimes, consorts like Asherah were associated with YHWH. Second Temple Jewish literature (Hebrew Bible, apocrypha, pseudepigrapha) reflects ideas of multiple divine or quasi-divine figures alongside God. Examples include the "one like a human being" or Son of Man in the Book of Daniel (possibly an elevated angel like Michael), personified Wisdom (seen as God's creation, child, or emanation), a divinized human in a Qumran hymn, the "Son of God" in the Daniel Apocryphon and Fourth Ezra, the "firstborn" angel Israel in the Prayer of Joseph, and Philo's concept of the Logos. These concepts provided a pool of ideas that early Christianity drew upon, particularly in developing its understanding of Jesus' divinity (Christology).
Binitarian ideas, implying two divine authorities side by side, continued to exist in later Jewish tradition, especially in Babylonian rabbinic Judaism and early Jewish mysticism (Hekhalot literature), even though they were often contested and rejected by mainstream rabbinic circles. In the Babylonian Talmud, there's a debate attributed to Rabbi Aqiva interpreting the plural "thrones" in Daniel 7:9 as meaning two thrones, one for God and one for the Messiah-King David, though this interpretation was strongly opposed by other rabbis. Early Jewish mysticism, particularly the Hekhalot literature, explicitly presented figures elevated to a godlike status, such as Enoch transformed into the angel Metatron, who is called the "Younger" or "Lesser God". This suggests a dynamic relationship between early Christianity and rabbinic Judaism, who can be seen as "sister religions" in ongoing exchange and even competition regarding these ideas of divine figures.
Introduction: Two Gods In Heaven
The Son Of Man In Daniel's Vision
The Personified Wisdom in the Wisdom Literature
The Divinized Human in the Self-Glorification Hymn from Qumran
The Son of God and Son of the Most High in the Daniel Apocryphon from Qumran
The Son of Man: Enoch in the Similitudes of the Ethiopic Book of Enoch