Being, existence, and unity are separate but related in Thelemic metaphysics.
Being and existence are caused by Nuit and Hadit respectively. They are similar to the mind of God and the will of God in Thomas Aquinas’s philosophy. The most obvious difference between Thelemic theology and Thomist theology is that Thelemic theology is polytheistic. One’s star is the only god one need concern oneself with from a Thelemic perspective, and with that caveat, Nuit and Hadit are the causes of being and existence respectively (within one’s own incommensurable universe which has one’s true self at the center).
The way in which Nuit and Hadit ground being and existence is by providing their respective unities. The unity of being is different from the unity of existence. Being or the “whatness” of things expresses itself through essences, which are always general and hierarchical. Essences are never unique to individuals; they are always shared by multiple beings. And they are understood in hierarchical genus-species relationships, in which beingness as such serves as the master genus. Thus when we are considering something through its being (or what is the same, its form), we can never separate it in reality from the sum total of being, of which Nuit is the ultimate cause.
Existence on the other hand accounts for the uniqueness of the individual. It does not answer “what” something is but rather “who” it is. It exists outside of hierarchy and can only ever be understood through itself, never in relation to a prior given whole.
The separation and cooperation of these two principles give Thelemic theology its distinctive character, which is both polytheistic and polycentric. If beingness had priority over existence, then the individuality of stars would be merely illusory. All “whos” would reduce to “whats”. If existence had priority over beingness, then (so Crowley argues) Thelema would be a form of Advaita Vedanta in which the first principle was consciousness without quality (what he represents as 1°). But because being and existence are equiprimodial, ultimate reality in Thelema expresses itself as a manifold of gods, each with their own wills. The pluralism of Thelema is underwritten by its polycentric theology.
What beingness and existence have in common in Thelema is that they are both causes of unity. Their mutual cause—unity itself—can neither be being nor existence, because then it would not have the power to cause being or existence. It must transcend both. This first cause of unity Crowley calls the Qabalistic Zero or 0°. No content can or should be thought through 0°. The most that can be said is that it is the quality of being-a-unit or unit-y itself. (If not, it wouldn’t be expressed as a mathematical unit.)
The Qabalistic Zero in and of itself is not the unity of anything. If it were, then Thelema’s theology would not be polycentric and polytheistic. It would be monotheistic. Each star would not be supreme, a law unto itself. It would merely be a more-or-less illusory spoke or epiphenomenon of whatever the more primordial reality was. Liberation would not consist of understanding itself and its own will but rather whatever that more general overarching cosmic law or dharma was. It is only because the first cause is unity plain and simple rather than the unity of something that Crowley can make statements like this:
The unveiling is the Proclamation of the Truth previously explained, that the Body of Nuith occupies Infinite Space, so that every Star thereof is Whole in itself, an independent and absolute Unit. They differ as Carbon and Calcium differ, but each is a simple “immortal” Substance, or at least a form of some simpler Substance. Each soul is thus absolute, and ‘good’ or ‘evil’ are merely terms descriptive of relations between destructible combinations.
New Comment on AL I.5
This is a great and holy mystery. Although each star has its own number, each number is equal and supreme. Every man and every woman is not only a part of God, but the Ultimate God. ‘The Centre is everywhere and the circumference nowhere’. The old definition of God takes new meaning for us. Each one of us is the One God
New Comment on AL I.4
Because unity is the first principle in Thelema (rather than being, thought, action, etc.), that arguably makes Thelema a kind of Platonism.
I explore these ideas in more detail in Thelema, Polytheism, and Absolute Individuality, but this post should help simply things a bit.