For most of European history the Hermetic writings were believed to be genuinely ancient Egyptian, the work of a sage contemporary with Moses or earlier, whose anticipation of Christian doctrine proved that a true theology had been given to the gentiles before the Greeks. Ficino took up the Corpus Hermeticum in 1462, interrupting his work on Plato to translate it for Cosimo de' Medici because the Hermetic texts were understood to be older and therefore more authoritative. That belief is the foundation of the entire Renaissance Hermetic project.
In 1614 Isaac Casaubon demolished it on philological grounds, in a book aimed at Baronio's church history rather than at the occultists. His argument was about language: the Greek is Koine, not the Greek of Plato's period or earlier, and the vocabulary is too recent for the claimed date, so the texts must be later than Plato rather than older than Moses. He placed them in the third and fourth centuries. The modern consensus, Brian Copenhaver's edition is the standard reference, puts them roughly between the first and third centuries CE, as products of Greek-speaking Egypt drawing on Platonism, Stoicism and Judaism.
One qualification matters, because Casaubon was right about the language and overreached in what he inferred from it. A late Greek date does not make the texts a Greek fabrication with nothing Egyptian in them; later scholarship, Garth Fowden's in particular, has recovered a real Egyptian religious substrate underneath the Greek philosophical surface. The tradition was wrong about the date by well over a thousand years, and not wrong about there being something Egyptian in it.
The interesting question is what the dating did, and the answer is: less than it should have. Serious scholarship abandoned the ancient Hermes, and the esoteric tradition largely did not. Occult writing still routinely dates the Emerald Tablet to remote antiquity, it is Arabic, not attested before about the eighth century, and reached Europe in Latin in the twelfth, and still describes Hermes as an Egyptian priest-king.
Hermes Trismegistus, thrice-greatest, is a syncretic figure of Greco-Roman Egypt, formed by the identification of Greek Hermes with Egyptian Thoth. He is not a person and never was one. He is the name under which alchemy, astrology and theurgy claimed an authority older than Greece, which is a real and important role, it is simply not a biography.