Hermes Trismegistus (Ancient Greek: Ἑρμῆς ὁ Τρισμέγιστος, "thrice-greatest Hermes"; Latin: Mercurius ter Maximus) is the purported author of the Hermetic Corpus, a series of sacred texts that are the basis of Hermeticism.
Many Christian writers, including Lactantius, Augustine, Marsilio Ficino, Campanella, and Giovanni Pico della Mirandola, as well as Giordano Bruno, considered Hermes Trismegistus to be a wise pagan prophet who foresaw the coming of Christianity.
They believed in the existence of a prisca theologia, a single, true theology that threads through all religions. It was given by God to man in antiquity and passed through a series of prophets, which included Zoroaster and Plato.
In order to demonstrate the verity of the prisca theologia, Christians appropriated the Hermetic teachings for their own purposes.
By this account, Hermes Trismegistus was either a contemporary of Moses, or the third in a line of men named Hermes, i.e. Enoch, Noah, and the Egyptian priest king who is known to us as Hermes Trismegistus on account of being the greatest priest, philosopher, and king.
This last account of how Hermes Trismegistus received that epithet is derived from statements in the Emerald Tablet of Hermes Trismegistus, that he knows the three parts of the wisdom of the whole universe.
Another explanation, in the Suda (10th century), is that "He was called Trismegistus on account of his praise of the trinity, saying there is one divine nature in the trinity."
Hermes Trismegistus may be associated with the Greek god Hermes and the Egyptian god Thoth. Greeks in the Ptolemaic Kingdom of Egypt recognized the equivalence of Hermes and Thoth through the interpretatio graeca.
Consequently, the two gods were worshiped as one, in what had been the Temple of Thoth in Khemenu, which was known in the Hellenistic period as Hermopolis. Hermes, the Greek god of interpretive communication, was combined with Thoth, the Egyptian god of wisdom.
The Egyptian priest and polymath Imhotep had been deified long after his death and therefore assimilated to Thoth in the classical and Hellenistic periods. The renowned scribe Amenhotep and a wise man named Teôs were coequal deities of wisdom, science, and medicine; and, thus, they were placed alongside Imhotep in shrines dedicated to Thoth–Hermes during the Ptolemaic Kingdom.
A Mycenaean Greek reference to a deity or semi-deity called ti-ri-se-ro-e (Linear B: 𐀴𐀪𐀮𐀫𐀁; Tris Hḗrōs, "thrice or triple hero") was found on two Linear B clay tablets at Pylos and could be connected to the later epithet "thrice great", Trismegistos, applied to Hermes/Thoth.
On the aforementioned PY Tn 316 tablet—as well as other Linear B tablets found in Pylos, Knossos, and Thebes—there appears the name of the deity "Hermes" as e-ma-ha (Linear B: 𐀁𐀔𐁀), but not in any apparent connection with the "Trisheros". This interpretation of poorly understood Mycenaean material is disputed, since Hermes Trismegistus is not referenced in any of the copious sources before he emerges in Hellenistic Egypt.
Cicero enumerates several deities referred to as "Hermes": a "fourth Mercury (Hermes) was the son of the Nile, whose name may not be spoken by the Egyptians"; and "the fifth, who is worshiped by the people of Pheneus [in Arcadia], is said to have killed Argus Panoptes, and for this reason to have fled to Egypt, and to have given the Egyptians their laws and alphabet: he it is whom the Egyptians call Theyt".
The most likely interpretation of this passage is as two variants on the same syncretism of Greek Hermes and Egyptian Thoth (or sometimes other gods): the fourth (where Hermes turns out "actually" to have been a "son of the Nile," i.e. a native god) being viewed from the Egyptian perspective, the fifth (who went from Greece to Egypt) being viewed from the Greek-Arcadian perspective.
Both of these early references in Cicero (most ancient Trismegistus material is from the early centuries AD) corroborate the view that Thrice-Great Hermes originated in Hellenistic Egypt through syncretism between Greek and Egyptian gods (the Hermetica refer most often to Thoth and Amun).