Who was Hermes Trismegistus, and what exactly is Hermeticism?

First and foremost, Hermeticism is a set of beliefs and systems composed by an unknown  set of individuals who used the name “Hermes Trismegistus” as their pseudonym in order that their esoteric teachings would be preserved throughout time. “Hermes Trismegistus” was a being of legend (lit. “Hermes thrice greatest”), whose namesake came into existence in order to reflect the cultural equivalents of a similar god/saint of writing: Hermes (Greek), Thoth (Egyptian) and Enoch (Hebrew). Amidst the body of works attributed to him (e.g. The Kyballion), the general philosophy of Hermeticism reflects an altogether unique way of thinking. In short, the story goes that the Divine, all-one, being created the entire Universe and loved its creation so much that it decided to live inside of it, and this explains how our own, individual divine natures came into being. We all have a God/desshead, and that is precisely this which gives us our freedom of will.

This engraving comes from 'Symbola aureae' by Michael Maier, published at Frankfurt in 1617. It famously depicts a mantra written by Hermes "As above so below".