There is no evidence whatsoever that there was ever a person named Hermes Trismestigus, and definitely not in the form in which Hermetic writings present him.

As for the Corpus Hermeticum itself, it dates from Alexandria around 200 AD, and represents a kind of cultic strain of Neoplatonic mysticism.

It first enters Europe - as the 'Emerald Tablet' - with the translation of the Secretum Secretorum by the "Aristotle by the Doctor" in the thirteenth century as part of the wave of translations from Arabic.

This amazing book, constructed as an epistolary exchange between Alexander the Great and Aristotle, was thought - by, for example, Roger Bacon - to contain the 'secret teachings' of Aristotle which enabled Alexander to conquer the world through various magical means. Alexander, in a motif that would subsequently repeat itself in the history of occultism, is described 'finding' the tablet in an ancient tomb. Through the Secretum Sectorum, it circulated Europe, inspiring magicians and alchemists, but without any evidence that I am aware of, of instigating "Hermeticism'' as such.

The underlying idea that Hermes was extremely ancient, possibly the teacher of Plato, or perhaps even Moses, or Zoroaster, was in line with the notion that the world was 6,000 years old, as the Bible clearly shows. Furthermore, because of course the most important thing that ever happened in the world was Christianity, there was a thought that there was something like a "prisca theologia" containing in outline the shape of the Revelation to come, which inspired ancient sages, being inspired by God, had foreseen.

Typically, this kind of proto-historical thinking only really begins in the Renaissance - that is, the beginning of the end as far as medieval Christianity was concerned.

The first place that you see it is Gemistos, aka Plethon, this very unusual neo-pagan philosopher whose life spans the last century of Byzantium, and who has the great historical honor of reintroducing Plato to Europe. But Plethon doesn't mention Hermes. The first time you find him is in Ficino, who translated 'the' Corpus Hermiticum after 'it' was found in a Macedonian monastery, and indeed broke-off translating Plato for it, because, the idea was, Hermes being older, Plato represented a continuation, and to some extent distortion of the original Hermetic message, when of course the truth was exactly the opposite. That truth became clear in 1600 when Isaac Causabon conclusively dated the Corpus Hermiticum to around 200 AD.