Although the medieval philosopher and scholar Moses Maimonides is generally seen (with good reason) as the epitome of Jewish rationalism, and his thought the antithesis of mysticism, Adam Afterman—the author of a recent book on the idea of mystical union with the deity in Jewish texts—argues that Maimonides made an important contribution to the history of Kabbalah. (Interview by Alan Brill.)
Maimonides more than any other medieval Jewish thinker was instrumental in the development of forms of mystical paths that end in mystical union. Maimonides internalized into his vision of Judaism the basic Aristotelian formula of knowledge and union, which was used to explain the contemplative transformation of the human intellect into an angelic intellect. . . . [This idea later] was adopted [by Jewish mystics] to explain how a human can integrate or assimilate into the Godhead.
The idea is that spiritual transformation in this life leads to the soul’s integration into spiritual realms associated [in prior Jewish literature] with the world to come, and eventually with the Godhead itself. [The Maimonidean theory of knowledge] helped the kabbalists explain how a human can integrate into God and how God may integrate into the human.
I must stress that I don’t think Maimonides himself was a mystic. And I don’t think he thought that man can unite with God. But Maimonides developed a worldview that divided the universe into two realms—the material and the non-material metaphysical realm. The metaphysical realm is considered to be unified in itself as pure thought. Thus the religious path that leads us from material existence to [a sort of mental union with angelic beings] is at the same time a movement from multiplicity to unity, a transformation from the corporeal to the union of intellect.
Read more on Book of Doctrines and Opinions: https://kavvanah.wordpress.com/2017/02/06/adam-afterman-interview-mystical-union-in-judaism/