In 2002 the BBC did a poll and Aleister Crowley was rated as number 73 in the top most influential Britons.
Many rising stars of the time made positive remarks about Crowley including but not limited to Ozzy Osbourne.
During his early life he travelled the world joining many secret society's seeking out occult wisdom, he was a member of the Hermetic Order of the Golden Dawn and even became a leader of the Ordo Templi Orientis.
He founded his own religion Thelema which fell apart when Italian authorities deported him after the accidental death of one of his followers during one of his ceremonies.
He is well known for many of his writings on the occult including one of his publications called "The Book of the Law".
Crowley was in essence a Ceremonial Magician practising the art of magic that involves summoning demons and controlling them with your "will" in order to carry out your magical orders. This practice is very much alive today and many base heavily on Crowley's work.
Crowley wrote on a large variety of other subjects outside of the occult including fiction and poetry, in some conspiracy theories it even alleged that he was a spy for the British government but there is little evidence to back this up.
He went to university in 1895 to start a 3 year course in philosophy, at Trinity College, Cambridge but change to English literature instead. He was not to join the golden dawn occult society until 1898 after he met a chemist named Julian L. Baker where they discussed their interest in alchemy.