From “The Hieroglyphic tradition” (by Erik Iversen)
Particular importance became attached to two short passages in Plato stating, in accordance with a genuine Egyptian tradition, that ‘a certain Theuth’, that is, Thoth, the Egyptian god of learning and writing, had been the first to observe that ‘the infinity of sound’ could be divided up into distinctive elements such as vowels and consonants, and was therefore considered not merely the discoverer of the concept of letters, but also the inventor of writing as such. And from a further passage in Cicero, in which it is stated that ‘Mercurius [the Latin name of Hermes or Thoth] …was said to have provided the Egyptians with laws and letters’, Thoth came to be considered in Roman times not merely the true originator of the art of writing, but also the primordial legislator—in Christian circles the Egyptian Moses.
[…] in the so-called Corpus Hermeticum, a collection of syncretistic treatises dating from about the third century A.D., Egyptian ‘philosophy’ was, in the opinion of Neoplatonic scholars, found in its purest and most uncontaminated form. Purportedly written by Thoth, the Egyptian god of wisdom and letters, who under his Greek title of Hermes Trismegistus gave his name to the compilation, it represents a blend of oriental mysticism, Gnosticism, and Greek philosophy, which the ancients considered genuinely Egyptian, and akin to the esoteric knowledge taught in the Egyptian mysteries of Isis and Serapis.
— Quoted from: The Legacy of Egypt (edited by J.R.Harris) – Oxford University Press 1971; “The Hieroglyphic tradition” (by Erik Iversen)