Complex sentence: He had a deep impact on the magic of the Hermetic Order of the Golden Dawn and later Aleister Crowley, and it was largely through this impact that Lévi is remembered as one of the key founders of the twentieth century revival of magic.
Split into simpler sentences: He had a deep impact on the magic of the Hermetic Order of the Golden Dawn and later on the ex-Golden Dawn initiate Aleister Crowley. It was largely through the occultists inspired by him that Lévi is remembered as one of the key founders of the twentieth century revival of magic.