Hermeticism is a path of magic which has evolved from a diverse blending of influences during two key periods. The first period was during the Rennaissance, when Christians of a mystical bent combined newly unearthed Greek, Hebrew and Arabic mysticism with native Alchemy, Freemasonry, and heretical Christian Theurgy and Thaumaturgy. The second period was during the Victorian era, when colonization of orient brought back a variety of meditational techniques (including Tantra) which were combined with the previously mentioned influences and both Spiritism and Theosophy.
Arising from the muddle of secret organizations was the Hermetic Order of the Golden Dawn, a pseudo-masonic organization which claimed to be descended from the Rosicrucians. This organization harbored a good number of English aristocrats as members, as well as theologians, poets, and people still known primarily for their occult work. One such person was Aleister Crowley, who left the Order angrily and began work with Ordo Templi Orientis. Claiming to descend from the Knights Templar, this organization was largely centered in Germany and was focused primarily on Tantra (sexually induced trances.) Crowley added his own ideas to the mix, including his "Book of the Law", a kind of Holy Bible to the religion he founded called Thelema.
During roughly the same period, Crowley's students and contemporaries added an immense amount of public literature to Hermeticism; largely because the Hermetic Orders were splintering into factions. New organizations devised new material in a flurry of inspiration, and individuals published the previously secret papers of dying Orders for the sake of posterity. During this period came forth individuals like Austin Osman Spare and Kenneth Grant, whose ideas on Hermetica go off into controversial directions (and border on, or are an early form of Chaoticism.) Nevertheless, the history of Hermeticism is extremely complicated and highly disputed.