Hermes Trismegistus (Greek "thrice-great Hermes"; Latin: Mercurius ter Maximus) is the representation of the syncretic combination of the Greek god Hermes and the Egyptian god Thoth. In Hellenistic Egypt, the Greeks recognised the congruence of their god Hermes with the Egyptian god Thoth. Subsequently the two gods were worshipped as one in what had been the Temple of Thoth in Khemnu, which the Greeks called Hermopolis.
Both Thoth and Hermes were gods of writing and of magic in their respective cultures. Thus, the Greek god of interpretive communication was combined with the Egyptian god of wisdom as a patron of astrology and alchemy. In addition, both gods were psychopomps; guiding souls to the afterlife. And there is also a connection with the Egyptian Priest and Polymath Imhotep.
A Mycenaean Greek reference found on a Linear B clay tablet at Pylos to a deity or semi-deity called TI-RI-SE-RO-E, Trisheros (the "thrice or triple hero") could be connected to the later epithet "thrice wise Trismegistus", applied to Hermes/Thoth. On the same Tn 316 tablet as well as other Linear B tablets, found in Pylos and Knossos, appears the name of the deity "Hermes" as E-MA-A, 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.
The origin of the description Trismegistus or "thrice great" is unclear. Copenhaver reports that this name is first found in the minutes of a meeting of the council of the Ibis cult, held in 172 BCE near Memphis in Egypt. Fowden however asserts that the earliest occurrence of the name was in the Athenagora by Philo of Byblos circa 64-141 CE. Another explanation is that the name is derived from an epithet of Thoth found at the Temple of Esna, "Thoth the great, the great, the great." The date of his sojourn in Egypt in his last incarnation is not now known, but it has been fixed at the early days of the oldest dynasties of Egypt, long before the days of Moses. Some authorities regard him as a contemporary of Abraham, and some Jewish traditions go so far as to claim that Abraham acquired a portion of his mystical knowledge from Hermes himself (Kybalion).
Many Christian writers, including Lactantius, Augustine, Giordano Bruno, Marsilio Ficino, Campanella and Giovanni Pico della Mirandola considered Hermes Trismegistus to be a wise pagan prophet who foresaw the coming of Christianity. They believed in a 'Prisca Theologia', the doctrine that a single, true, theology exists, which threads through all religions, and which 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 'priscia theologia' Christians appropriated the Hermetic teachings for their own purposes. By this account Hermes Trismegistus was either, according to the fathers of the Christian church, 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, or "thrice great" on account of being the greatest priest, philosopher and king.
This last account of how Hermes Trismegistus received the appellation "Trismegistus," meaning "Thrice Great," is derived from statements in the The Emerald Tablet of Hermes Trismegistus, that he knows the three parts of the wisdom of the whole universe. The three parts of the wisdom are alchemy, astrology, and theurgy. The pymander, from which Marsilio Ficino formed his opinion, states that "they called him Trismegistus because he was the greatest philosopher and the greatest priest and the greatest king."
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.
Modern occultists suggest that some Hermetic texts may be of Pharaonic origin, and that the legendary "forty-two essential texts" that contain the core Hermetic religious beliefs and philosophy of life remain hidden in a secret library.
In some trance "readings" of Edgar Cayce, Hermes or Thoth was an engineer from the submerged Atlantis, who also built, designed or directed the construction of the Pyramids of Egypt.
The book Kybalion, by authors dubbed "The Three Initiates," addresses Hermetic principles.
Within the occult tradition, Hermes Trismegistus is associated with several wives, and more than one son who took his name, as well as more than one grandson. This repetition of given name and surname throughout the generations may at least partially account for the legend of his longevity, especially as it is believed that many of his children pursued careers as priests in mystery religions.