There was no Hermetic church, no Hermetic priesthood and no Hermetic creed. There were texts, and the tradition is defined by them.
The core is the Corpus Hermeticum, seventeen Greek treatises assembled at some point in the Byzantine period out of a much larger literature. Most take the form of a dialogue in which Hermes Trismegistus, thrice-greatest Hermes, instructs a pupil, often Asclepius or his son Tat, on god, the cosmos, mind and the fate of the soul. The first and most famous, the Poimandres, describes a vision of the origin of things and the descent of the human being through the planetary spheres. Alongside the Corpus stand the Latin Asclepius, a longer dialogue surviving complete only in translation, the excerpts preserved by the anthologist Stobaeus, and Coptic Hermetic pieces found among the Nag Hammadi codices, which showed that this literature circulated in the same Egyptian milieu as the gnostic texts.
Hermes Trismegistus is a fusion. The Egyptian Thoth, god of writing, measurement and wisdom, was identified with the Greek Hermes in the Hellenistic period, and the resulting figure is treated in these texts less as a god than as an ancient Egyptian sage who had received revelation and written it down. That framing mattered enormously later, because it presented the writings as far older than they are.
The literature divides roughly in two. The philosophical Hermetica are the dialogues on god and soul; the technical Hermetica are the much bulkier body of astrological, alchemical and magical writings circulated under the same name, dealing with the properties of stones and plants, decans, talismans and the drawing down of celestial power. The older assumption that these served entirely separate audiences has weakened. The Asclepius describes the animation of statues, and the philosophical texts share their cosmology with the technical ones.
Dating is now reasonably settled: the surviving Greek treatises were composed in Roman Egypt, mostly in the second and third centuries CE, by authors writing in Greek and drawing on Platonism, Stoicism and some genuinely Egyptian material. What they are not is a survival of pharaonic religion, and the whole later history of Hermeticism turns on how long it took Europe to work that out.