The religion known as Thelema was established with the writing of The Book of the Law.
The full title of this book is:
Liber AL vel Legis
sub figura CCXX
The Book of the Law
as delivered by XCIII=418 to DCLXVI
It was written (or received) by Aleister Crowley in Cairo, Egypt in the year 1904. It contains three chapters, each of which was written down in one hour, beginning at noon, on April 8th, 9th, and 10th. Crowley claims that the author was an entity named Aiwass, whom he later identified as his own Holy Guardian Angel. The teachings within this small book are clearly expressed in the Law of Thelema, expressed by these two phrases:
“Do what thou wilt shall be the whole of the Law” (AL I:40)
“Love is the law, love under will” (AL I:57)
Interpretation of this book is considered a matter for the individual, and openly promoting personal ideas about its meaning is strongly discouraged. Although Crowley expressed the desire to see the Law of Thelema promulgated in all areas of society, success in this endeavor is to be based on others willfully following the the good example of Thelemites rather than by evangelization or direct attempts to convert others.
“Success is thy proof: argue not; convert not; talk not over much!” (AL III:42)
You can read the The Book of the Law on the Grand Lodge website.