Thelema is the English transliteration of the Ancient Greek noun θέλημα: "will", from the verb θέλω: to will, wish, purpose. Early Christian writings use the word to refer to the will of God, the human will, and even the will of God's opponent, the Devil.

Thelema is also an initially fictional philosophy of life first described by François Rabelais (16th century) in his famous books, Gargantua and Pantagruel. The essence of this philosophy was summarized in the phrase "fay çe que vouldras" ("Fais ce que tu veux," or, "Do what thou wilt"), and this philosophy was later put into practice in the mid 18th century by Sir Francis Dashwood at Medmenham.

This Thelemic Law of Rabelais was revived by Aleister Crowley in 1904 when Crowley wrote The Book of the Law, which contains both the word Thelema in Greek as well as the phrase "Do what thou wilt." From this, Crowley took Thelema as the name of the philosophical, mystical and religious system which he subsequently developed, which includes ideas from occultism, Yoga, and both Eastern and Western mysticism especially the Qabalah. Thus Shri Gurudev Mahendranath, in speaking of svecchachara, a Sanskrit term which he considered the eastern equivalent of the term Thelema, wrote that "Rabelais, Dashwood, and Crowley must share the honor of perpetuating what has been such a high ideal in most of Asia."

In 1904, Aleister Crowley (1875–1947) — an English occultist, writer, and social provocateur— claimed to have received and written down The Book of the Law, which was to serve as the foundation of the religious and philosophical system he called Thelema, so named, according to his friend of the 1930s, Shri Gurudev Mahendranath, "in reverence to the Rabelaisian masterpiece." He summed up the Law of Thelema in these phrases from the Book:

"Do what thou wilt shall be the whole of the Law"

"Love is the law, love under will"

There is no Law beyond Do what thou wilt"

Crowley wrote that the Law is not a license to indulge in casual whim or to mindlessly accept cultural mores, but is rather a mandate to discover and manifest one's True will, which he described as one's inner divine nature, spiritual destiny, or proper course in life.

 The Book of the Law

 True Will

According to Crowley, the discovery and manifestation of one's unique True Will is the central task of every Thelemite. True Will is an idea that could be described in its dynamic aspect as the singular path of possible action that encounters no resistance in going because it is supported by the inertia of the whole Universe; theoretically, no two True Wills can contradict each other because each one has its own absolutely unique career in its passage through Infinite Space. Hence, to follow one's True Will means to respect all True Wills, described as "Love is the law, love under will". The apparent pacifism of this doctrine is complicated, however, by the possibility that the majority of beings do not know their True Will.

Crowley referred to the process of discovering the Will as the Great Work, the basis of which is Love or Union with the All (similar in vein to the mystical aspects of Buddhism and Hinduism). The term Magick is applied to the general set of techniques used to accomplish the Great Work, which usually includes practices based on Yoga, the Qabalah, Hermeticism, and ceremonial ritual. According to Crowley, the two great milestones in this process are attaining the Knowledge and Conversation of one's Holy Guardian Angel (which Crowley described as a person's "Secret Self") and then crossing the Abyss, a mystical process where the individual ego is "annihilated" (symbolized by the spilling of the blood into the Graal of Babalon) and the adept achieves union with the All by entering the City of the Pyramids. After this, the "Master of the Temple" may either remain there, move on to higher states, or return to every-day life to fulfill some earthly destiny. (See also: Thelemic mysticism)

 Cosmology

The Stele of Revealing, depicting Nuit, Hadit as the winged globe, Ra-Hoor-Khuit seated on his throne, and the creator of the Stélé, the scribe Ankh-af-na-khonsu

1. Babalon—the Scarlet Woman, the Mother of Abominations, the Holy Whore
2. Chaos—the universal generative drive
3. Baphomet—the Serpent and the Lion, creative energy materialized
4. Aiwass—the being that, according to Crowley, dictated Liber AL vel Legis, and whom Crowley claimed to be his own Holy Guardian Angel
5. Ankh-f-n-khonsu—an actual Priest who lived in Thebes during the late XXVIth Dynasty of ancient Egypt, around 725 BCE.

 Magick

Crowley saw magick as the essential method for a person to reach true understanding of the self and to act according to one's True Will. In the broadest sense, magick is any act designed to cause intentional change. It is not capable of producing "miracles" or violating the physical laws of the universe (i.e. it cannot cause a solar eclipse), although "it is theoretically possible to cause in any object any change of which that object is capable by nature" (Book 4). Crowley describes the general process:

"One must find out for oneself, and make sure beyond doubt, "who" one is, "what" one is, "why" one is...Being thus conscious of the proper course to pursue, the next thing is to understand the conditions necessary to following it out. After that, one must eliminate from oneself every element alien or hostile to success, and develop those parts of oneself which are specially needed to control the aforesaid conditions." 

For Crowley, the practice of magick—although it equally applies to mundane things, like balancing the checkbook—is essentially to be used for attaining the Knowledge and Conversation of one's Holy Guardian Angel. Since achieving this state with one's 'Silent Self' can be extremely arduous, magick can be used not only to reach that particular goal, but to clear the way for it as well. For example, if one needed a particular dwelling to perform the operation, one could use magick to obtain a suitable home. Crowley stated that magick that did not have one of these goals as its aim was black magic and should be avoided.

 Practices and observances

Although there are communal ceremonies informed by Thelema and organizations to support them (see Thelemic organizations), Thelemic practice is largely an individual affair. Generally, practices are designed to assist the Thelemite in finding and manifesting True Will, although some include celebratory aspects as well.

Crowley wrote many rituals and discussed numerous spiritual practices that he considered central to the Thelemic experience. These include (but are certainly not limited to):

1. Liber Resh—consisting of four daily adorations to the sun
2. The purification, consecration and exaltation of one's Body of Light by the use of rituals of invocation (e.g. the Ritual of the Pentagram)
3. Eucharistic celebrations, such as The Gnostic Mass or the Mass of the Phoenix
4. Development in Yoga
5. Keeping a magical record (a sort of diary for recording ritual and mystical experiences)
6. "Saying Will" before the main meal of the day (a simple set of statements—sometimes presented as a dialog with others—declaring that it is the individual's will to eat and drink, in order to fortify his body, in order to accomplish the Great Work.)

 Ethics

Thelema stresses individual liberty balanced by responsibility and discipline, the inherent divinity of every person, regardless of gender , and the battle against superstition and tyranny. Ultimately, the interpretation of Thelema and The Book of the Law is left to the individual; for this reason, aggressive attempts at conversion are strongly frowned upon, although using personal example to promulgate the Law is encouraged .

Crowley wrote two documents to codify his concept of Thelemic ethics: Oz and Duty.

 Liber Oz

Liber Oz establishes the rights of the individual. For each person, these include the right to: live by one's own law; live in the way that one wills to do; work, play, and rest as one will; die when and how one will; eat and drink what one will; live where one will; move about as one will; think, speak, write, dress, love, paint, carve (etc.) as one will; and kill those who would thwart these rights. The rights established in Oz are often considered to be complemented by the obligations given in Duty.

 Duty

Duty is described as "A note on the chief rules of practical conduct to be observed by those who accept the Law of Thelema." There are four sections:

1. Duty to Self: essentially describes the self as the center of the universe, with a call to learn about one's inner nature. Further, every Thelemite is to develop every faculty in a balanced way, establish one's autonomy, and to learn and do one's True Will.
2. Duty to Others: A Thelemite is called to eliminate the illusion of separateness between oneself and all others, to fight when necessary, to avoid interfering with the Wills of others, to enlighten others when needed, and to recognize the divine nature of all other beings. Further, it is noble to relieve the suffering of others, but pity (seen as condescending) should be avoided.
3. Duty to Mankind: Thelemites should try to establish the Law of Thelema as the sole basis of conduct. Further, the laws of the land should have the aim of securing the greatest liberty for all individuals. Crime is viewed from the point of view of violating one's True Will ("Thus, murder restricts his right to live; robbery, his right to enjoy the fruits of his labour; coining, his right to the guarantee of the state that he shall barter in security; etc.").
4. Duty to All Other Beings and Things: Quite simply: "It is a violation of the Law of Thelema to abuse the natural qualities of any animal or object by diverting it from its proper function" and "The Law of Thelema is to be applied unflinchingly to decide every question of conduct."

 Diversity of Thelemic thought

The core of Thelemic thought is "Do what thou wilt." However, beyond this, there exists a very wide range of interpretation of Thelema. For example, some organizations and persons regard Crowley's system to be only one possible manifestation of Thelema, choosing to borrow instead from Rabelais or other original system. Others accept The Book of the Law in some way, but not the rest of Crowley's "inspired" writings or teachings. Yet others take only specific aspects of his overall system—such as his magical techniques, ethics, mysticism, or religious ideas—while ignoring the rest. Considering the strong emphasis on the unique nature of Will inherent in each individual, it is perhaps inevitable that many Thelemites would tend to avoid strongly dogmatic or "fundamentalist" thinking. Crowley himself (at times) supported this view:

I admit that my visions can never mean to other men as much as they do to me. I do not regret this. All I ask is that my results should convince seekers after truth that there is beyond doubt something worth while seeking, attainable by methods more or less like mine. I do not want to father a flock, to be the fetish of fools and fanatics, or the founder of a faith whose followers are content to echo my opinions. I want each man to cut his own way through the jungle. 

Many adherents of Thelema are syncretic and recognize correlations between Thelemic and other systems of spiritual thought; most borrow freely from other traditions. For example, Nu and Had are thought to correspond with the Tao and Teh of Taoism, Shakti and Shiva of the Hindu Tantras, Shunyata and Bodhicitta of Buddhism, Ain Soph and Kether in the Qabalah. Adherents of Thelema, none so more than Crowley, make free use of the methods and practices derived from other traditions, including alchemy, astrology, qabalah, tantra, tarot, and yoga, regarding them all as being subsumed within Thelema.

While some organizations attempt to stay true to Crowley's system (such as O.T.O. and the A∴A∴), several organizations deviate from his core teachings, in some cases substantially. For example, the Fraternitas Saturni (Brotherhood of Saturn), founded in 1928 in Germany, accepts the Law of Thelema, but extends it with the phrase "Mitleidlose Liebe!" ("Compassionless love!"). The Thelema Society, also located in Germany, accepts Liber Legis and much of Crowley's work on magick, while incorporating the ideas of other thinkers, such as Friedrich Nietzsche, Charles S. Peirce, Martin Heidegger, and Niklas Luhmann. In America, the writings of Maggie Ingalls (Nema) have inspired a movement called Maat Magick, along with an organization called the Horus-Maat Lodge, founded in 1979. This movement combines Crowley's essential elements of Thelema with Nema's system based on the Egyptian goddess Ma'at, as established in her received work, "Liber Pennae Praenumbra." HML aims to combine the current Aeon of Horus with the future Aeon of Ma'at, where the combined mind of humanity will awaken and mankind will achieve balance.

 Contemporary Thelemic literature

By far, the bulk of Thelemic writing remains that of Aleister Crowley. He was highly prolific and wrote on the subject of Thelema for over 35 years, and many of his books remain in print. During his time, there were a few who wrote on the subject, including Charles Stansfeld Jones and J.F.C. Fuller. Since his death in 1947 only a few Thelemic voices have appeared in published books. Perhaps the four most published voices have been:

• Israel Regardie, who not only edited many of Crowley's works, but wrote a biography of him—The Eye in the Triangle—and penned many books on ritual and Qabalah, such as Garden of Pomegranates, Golden Dawn, Middle Pillar, and Tree of Life.
• Kenneth Grant, who has written many books on Thelema and the occult, such as The Magical Revival, Aleister Crowley and the Hidden God, Outside the Circles of Time, and Hecate's Fountain.
• Lon Milo DuQuette, a popular author whose books are mostly dedicated to analyzing and exploring Crowley's system, including such books as Understanding Aleister Crowley's Thoth Tarot, The Chicken Qabalah of Rabbi Lamed Ben Clifford, The Magick of Aleister Crowley, and The Key to Solomon's Key.
• Nema, whose Liber Pennae Praenumbra announces and explains the Ma'atian current has influenced Thelemites for over 25 years. She now has several books on Ma'atian Thelema including her book, Maat Magick.

Other notable contemporary writers who address Thelema include Jerry Edward Cornelius, Christopher Hyatt, and Jack Parsons.

There are also numerous publications that print original Thelemic writing, such as the journals Light In Extension, Lion & Serpent, The Scarlet Letter, and Cornelia.

 Thelemic organizations

Several modern organizations of various sizes claim to follow the tenets of Thelema. The two most prominent are both organizations that Crowley headed during his lifetime, the A∴A∴—a teaching order designed to guide initiates through Crowley's mystical system of Thelema—and Ordo Templi Orientis—a fraternal order that initially developed from freemasonry and includes the Gnostic Catholic Church (which celebrates the Gnostic Mass).

Since Crowley's death in 1947, other organizations have formed that try to carry on his initial work—for example, Phyllis Seckler's College of Thelema, the Ordo Templi Orientis of Kenneth Grant, Society O.T.O. of Marcelo Ramos Motta, OTO Foundation, Thelemic Order of the Golden Dawn, the Holy Order Of Ra-Hoor-Khuit, The Open Source Order of the Golden Dawn, and The Order of Thelemic Knights. Other groups of widely varying character exist which have drawn inspiration or methods from Thelema, such as the Illuminates of Thanateros and the Temple of Set. Groups such as Fraternitas Saturni, Horus-Maat Lodge, The Hawk and Jackal Covens, and The Thelema Society accept the Law of Thelema, but omit certain aspects of Crowley's system while incorporating the works of other mystics, philosophers, and religious systems.

 Thelema in comparative religion

Thelema has been attracting more attention in recent years from scholars of religion, especially those interested in new religious movements, contemporary Gnosticisms and Hermeticisms. References at the end of this article supply a few such sources. Perhaps the most unusual attempt was made by bishop Federico Tolli, in his German book Thelema — Im Spannungsfeld zwischen Christentum, Logentradition und New Aeon  For Tolli, Thelema is to be regarded as the dialectical consequence of Christianity. Christianity for Tolli exists as a community in Christ, whereas Tolli sees Thelema as a necessarily individualistic response to the world.

Taken from a 1938 theological dictionary (to the New Testament), the concept of 'salvation history' (Heilsgeschichte) has a great effect on Tolli's thought, and it is in this context that he discusses Crowleyan Thelema. Tolli regards Crowley's Heilsgeschichte as one in which the whole Universe (ergo the Will of God) is to combine (analogous to the Alchemical formula 'coagula'). "Love", in the form of combinatory attraction ("Love is the law, love under will"), is a universal principle — therefore akin to the concept of Natural religion. The main difference (for Tolli) is that in Christianity salvation of the entire Universe ("Ganzheit") cannot be made by 'solipsistic' man. The bishop sees Crowley as a failed – however talented – artist or "Mystagogie", but not as a "Satanist". The merit and contribution of bishop Tolli to Thelemic studies lies in the fact that it was he who first expresses that the genuine meaning and idea of Thelema does not necessarily contradict the teachings of Jesus, as Crowley himself affirms.