The Invisible God is abstract, the ultimate reality in the universe. It is Brahman, the Ein Sof, the Unmoved Mover, the One. The Invisible God does not change, but is the cause of all. The Invisible God is unity, undivided, infinite, and the single binding truth behind diversity in the universe.
Next to the Invisible God, all others are infinitely small, localized, and insignificant. They may be called on or used if necessary, but the Invisible God is the All.
The zzaburi are, among other things, the “priests” of the Invisible God. The Invisible God provides no Rune magic, not spirits, or Divine Intervention. But as the ultimate reality in the universe, study and contemplation of the Invisible God gave rise to sorcery, a rational system that allows mortals to understand and command the natural laws of the cosmos.
So one way of thinking about the zzaburi is that they are brahmins, performing Brahma-yajna before the altar of the Invisible God, surrounded by geometric mandalas that depict cosmological mathematics. They chant, sing, and recite, while the rest of the community come by and offer gifts and obeisance.
I think defining Malkionism raises some of the same problems as trying to define Hinduism. Malkionism includes a diversity of ideas on spirituality and tradition, and within Malkionism are polytheists, pantheists, pantheists, henotheists, monotheists, and even atheists. Even caste is a subject of debate within Malkionism. Maybe it is merely the presence of sorcerer-priests of the Invisible God that makes it Malkionism? Maybe not even that?
But one way of thinking about the zzaburi is to analogise them to brahmins. But if you start taking that literally, then you’ve taken the analogy too far.