Hermes Mercurius Trismegistus was referred to as the ‘thrice great,’ which is the translation of Trismegistus. Hermes received this title in the Twelfth Aphorism of The Emerald Tablets. In this text, he was said to have contained within him three aspects of all wisdom known to the world.
Hermes is most often considered synonymous with the Egyptian God, Thoth. In some accounts, he is considered to be the reincarnation of Thoth, while in others the two are one in the same having lived at the same time. It is unknown historically which of these, if either, are actually true.
After the time of Hermes and his forebear, Thoth, many wisdom schools adopted the tradition of titling every manuscript written in the Hermetic tradition to Hermes Mercurius Trismegistus. There are two texts known to certainly have been written by him, The Divine Pymander and The Virgin of the World.
Although there are two versions of The Emerald Tablets, one attributed to Hermes and the other to Thoth, the actual dating of the manuscript attributed to Hermes currently lies in about 1100 A.D., and remains uncertain as to its antiquity or authorship. The Emerald Tablets of Thoth are of much more recent origin (1925 A.D.), translated by a mysterious man who calls himself Doreal of the Great White Brotherhood. He claims to have found the 36,000-year-old texts in the early 20th century somewhere around the region of the Great Pyramid and that they were made of an indestructible alchemically altered emerald green stone.
Despite the uncertain origin of both versions of The Emerald Tablets, they contain a similar philosophy and theology to that of The Divine Pymander.
Surprisingly, the teachings contained within The Divine Pymander are very monotheistic despite the Egyptian Pantheon of Gods. Hermes Mercurcius Trismegistus describes God as Father, not unlike early Christian tradition and the writings of the Early Church Fathers, and in the tradition of many mystics since his time, he demonstrates a world containing many layers of light and darkness wherein the most important mission of each individual man is to purify himself in this world and the next to meet a high and holy God, a belief not dissimilar to the Catholic Doctrine of Purgatory:
“For this only, O Son, is the way to Truth, which our Progenitors traveled in; and by which making their journey, they at length attained to the good. It is a venerable way and plain, but hard and difficult for the soul to go in that is in the body. For first it must war against its own self, and after much strife and dissension, it must be overcome of one part; for the contention is of one against two, whilst it flies away, and they strive to hold and detain it. But the victory of both is not like, for the one hasteth to that which is Good, but the other is a neighbour to the things that are Evil; and that which is Good desireth to be set at liberty, but the things that are Evil love Bondage and Slavery.” The Divine Pymander of Hermes, Translated by Dr. Everard
Hermes has been given the honorable title of the father of chemistry in that he originated many of the earliest forms of alchemy and the chemical sciences, but there are many books attributed to Hermes Mercurius Trismegistus on theology, astronomy and medicine, as well.
Dr. Everard was a well-known minister who was believed to be removed from his pastoral position due to accusations of heresy.
Sources: The Divine Pymander of Hermes – Translated by Dr. Everard, The Emerald Tablets of Thoth the Atlantean – By Doreal, The Emerald Tablets of Hermes, Corpus Hermeticum and the Hermetic Library