As usual, it depends on how we define our terms. I’ll consider Crowley in relation to three kinds of naturalism described by Paul Franks in his book All or Nothing: methodological naturalism, metaphysical naturalism, and theological naturalism.
Methodological Naturalism
If a naturalist is someone who believes there is no need for a discipline distinct from natural science to acquire knowledge, or that all methods of acquiring knowledge are ultimately continuous with natural science, the answer is ambiguous.
Crowley believed it was possible to acquire knowledge by occult means. He summoned spirits and directed questions to them about human affairs. He utilized I Ching to make decisions. He regarded the Book of the Law and hence Thelema itself to be a revelation that proved what he called the postulate of religion. From this perspective, Crowley clearly believed there were methods of acquiring knowledge discontinuous with the methods of natural science (at least as currently understood).
On the other hand, he also seemed to believe natural science would one day be able to explain these methods naturalistically. This is reflected in the motto of the organ of the A∴A∴, The Equinox: the method of science, the aim of religion. All aspirants to the A∴A∴ were required to keep detailed records of their “experiments”. So as far as his intentions go, he was naturalistic in this sense, even if, a century later, most scientists would not regard many of his methods as scientifically valid.
But there is another sense in which Crowley rejects methodological naturalism. He believes metaphysical knowledge can be acquired through the application of mathematics alone, absent any particular empirical objects. In particular his ontology is based upon an a priori analysis of what he calls “nothingness extended in no categories.” So in this sense he is unambiguously not a naturalist, since his philosophical methodology is recognizable as what philosophers today refer to as a form of transcendental argumentation.
Metaphysical Naturalism
If a naturalist is someone who believes the only possible entities are the empirical objects studied in natural science, then Crowley is not a naturalist. Crowley believed conscious entities existed independently of spacetime and spatiotemporal causality. He thought Aiwass was an example of just such an entity. He also believed the human soul—what the Book of the Law refers to as the star or khabs—was such an entity. This latter view is reflected in Liber XV and the initiation rituals of O.T.O., and it finds expression in Liber Aleph, the commentaries on the Book of the Law, Liber CL, Magick in Theory and Practice, Magick Without Tears, and other works. So in this sense of the term—what we might term metaphysical naturalism, since it has to do with what exists—Crowley was unambiguously not a naturalist.
Naturalistic Theology
If a naturalist is someone whose metaphysical commitments are entirely secular, someone who makes no sectarian theological claims, then the answer is murky. No one can doubt that Crowley makes theological claims in many of his writings. The Book of the Law itself is spoken through the personae of three Egyptian deities. The mysteries of O.T.O. and A∴A∴ reflect Crowley’s interest, since his Golden Dawn days, in trinitarian theology.
But Crowley likely would not have thought that theology necessarily contradicts secularism, if by a secular claim we mean one that is ultimately grounded in a priori reasoning. In other words, if all his theological claims ultimately find their grounding in his ontology, that implies he thought the “truth” of all sectarian religious claims was ultimately secular.
This view that sectarian theological claims could ultimately be translated into rational secular terms was a common view in the late-18th, 19th, and early-20th centuries. It’s reflected in solar-phallicism, the occultism of Eliphas Levi, and it finds expression in the common mystical core thesis of the psychologist William James—all of whom influenced Crowley.
This form of secularizing naturalism is reflected in Liber XV, Crowley’s most important attempt to satisfy the “religious instinct”. It’s reflected in the curriculum of A∴A∴, which is agnostic with regard to both theological and practical approaches to religious truth. The student must study, practice, and be tested in ideas and methods from a variety of traditions. It’s reflected in the sense in which Crowley connects or even equates the attainments of ceremonial magick—in particular the Knowledge and Conversation of the Holy Guardian Angel—with attainments from the Christian, Muslim, Buddhist, and Hindu traditions.
Clearly Crowley’s intentions were naturalistic in this sense. The theological aspects of Thelema were intended to be “catholic”. He thought they would be accepted by most rational people. Empirically, that has not been the case.
Furthermore, since Crowley’s time, many of his theologically naturalistic assumptions are considered controversial (as in the case of the common core thesis of mysticism), or they have been largely ignored or discarded (perennialism and solar-phallicism). So even though Crowley’s intentions can be regarded as theologically naturalistic, theological naturalism is far less important to contemporary naturalistic investigations of religion than methodological naturalism. In other words, even if Thelema is naturalistic in this sense, it is naturalistic in a way which tends to be regarded as passé.
Conclusion
Crowley aspired to certain forms of naturalism. Methodological naturalism informs the work of the A∴A∴, and Crowley’s attempt to turn Thelema into a catholic religion rests upon his theological naturalism. It’s impossible to understand what Crowley was doing without understanding those underlying naturalistic motivations.
At the same time, very few naturalists today would probably consider Crowley one of their own in anything other than spirit. Insofar as he adopts a naturalistic methodology, it seems to be done with the agenda of proving dated beliefs regarding “discarnate” “Secret Chiefs,” reincarnation, and the magical efficacy of certain solar-phallic “secrets”. Crowley thought his and his students’ investigations would lead to discoveries that would revolutionize the human condition. In fact the naturalistic approach has dramatically changed the human condition since Crowley’s time, just not in the way Crowley was focused upon.
Nor has the naturalistic investigation of religion advanced in a way Thelema has been very relevant to. The perennialist, “catholic” assumptions of Thelema have largely been rejected in favor of empirical investigations of particular practices. Meditation and its effects have received a lot of attention. Boccia, Piccardi, and Guariglia recently did a meta-analysis of MRI studies on functional changes in the brain states of meditators. Rosch, Thompson, and Varela as well as John Vervaeke have proposed cognitive psychological models of meditation and its effects. But meditation has not historically been an important part of the Thelemic tradition, at least not since Crowley himself wrote about it. Nor has Thelema contributed much to our understanding of meditation and its effects in the 20th and 21st centuries as compared with Buddhism. Instead, ceremonial magick has been an important component of Thelema; it is indeed one of the distinguishing characteristics of the Thelemic tradition that it has actively developed ceremonial magick.
One of Vervaeke’s students at the University of Toronto, Juensung Kim, has been developing a cognitive psychological understanding of “esoteric practices,” such as magic, divination, and deity devotion. With his colleagues Abramovich, Long, and Morris, he has surveyed practitioners, and they have begun to look at the effects these practices have on growth, grounding, and porosity. This is very much unlike Crowley’s approach, proceeding as it does without perennialist assumptions of the ultimate reconcilation of eastern and western traditions—to say nothing of the metaphysical claims about discarnate entities Crowley was attempting to prove.
But ceremonial magick, divination, spirit invocation and evocation, and devotional practices continue to be prominent (even distinctive) components of the Thelemic spiritual tradition—similar to how mindfulness meditation has and continues to be a distinctive component of the western Buddhist tradition. This means Thelemic practitioners of these and related “esoteric practices” can help contribute to these new properly naturalistic investigations in important ways.