Thelema is a spiritual path, and a religion only in the broadest sense of the word. The central document is Liber AL vel Legis, or the Book of the Law. This book was narrated to Aleister Crowley on three consecutive days, April 8th, 9th, and 10th, 1904, by a supernatural being. This marked the ending of the old Aeon and the beginning of the new one.
The universe runs in Aeons, and the changing from one to the other is known as the Equinox of the Gods. First was the age of Isis, the mother. During this time, society and religion was matriarchal. The moon was worshiped, because humanity saw correlation between the lunar cycle and childbirth, and that women bore the children. Next came the age of Osiris, the father, and religion became patriarchal and sun-based, as people began to recognize the male's role in creating a child, and the sun's role in growing crops. In 1904, we entered the age of Horus, the child. As society and technology advance, we as humans become more and more child-like. This can be observed in wars, greed, and people's need for entertainment.
The central teaching of Thelema is “Do what thou Wilt shall be the Whole of the Law”, and second is “Love is the Law, Love under Will”. Thelema literally means 'Will'. The Law of Thelema telling us “do what thou Wilt” is not saying to do whatever you want. The Great Work of Thelema is to discover one's True Will, and then do nothing else. In every moment, we make choices, and only one of them is our Will. The rituals and practices in Thelemic training help us recognize our Will and do it.
Religion for many people is passive belief. Thelema is an active practice. The first word of the Law is DO. It is not enough to simply believe. In order to accomplish the Great Work, Thelemites must actually work for it.
Another thing many religions have in common, is that the beliefs focus on an end result, like going to heaven. With Thelema, the important aspect is the journey, not the destination. The path itself is the destination, and once you're on it, you never leave it.
Thelema is philosophical, spiritual, Magickal, and practical. While some look at it and view it as evil or satanic, that is their beliefs. For me, I've found my truth, and I would never force it on anyone. I hope I haven't offended anyone for sharing my beliefs.