In Hesiod's Theogony, Phaëton becomes a daimon, de-materialized, but the ills of mankind released by Pandora are keres not daimones. Hesiod connects the daimones of the deceased great and good in relating how the men of the Golden Age were transmuted into daimones by the will of Zeus, to serve as ineffable guardians of mortals, whom they might serve by their benevolence. In similar ways, the daimon of a venerated hero or a founder figure, located in one place by the construction of a shrine rather than left unburied to wander, would confer good fortune and protection on those who stopped to offer respect. Thus daemones ("replete with knowledge", "divine power", "fate" or "god") were not necessarily evil.
The Greek translation of the Septuagint, made for the Greek-speaking Jews of Alexandria, and the usage of daimon in the New Testament's original Greek text, caused the Greek word to be applied to a Judeo-Christian spirit by the early 2nd century AD. Then in late antiquity, pagan conceptions and exorcisms, part of the cultural atmosphere, became Christian beliefs and exorcism rituals. The transposition has recently been documented in detail, in North Africa, by Maureen Tilley.
Though in Homer the words θεοί (gods) and δαίμονες (divinities) were practically synonymous, later writers like Plato developed a distinction between the two. Plato in Cratylus (398 b) gives the etymology of δαίμονες (daimones) from δαήμονες (daēmones) (=knowing or wise), though in fact the root of the word is more probably daiō (=to distribute destinies). In Plato's Symposium, the priestess Diotima teaches Socrates that love is not a god, but rather a "great daemon" (202d). She goes on to explain that "everything daemonic is between divine and mortal" (202d-e), and she describes daemons as "interpreting and transporting human things to the gods and divine things to men; entreaties and sacrifices from below, and ordinances and requitals from above..." (202e). In Plato's Apology of Socrates, Socrates claimed to have a daimonion (literally, a "divine something") that frequently warned him - in the form of a "voice" - against mistakes but never told him what to do. However, the Platonic Socrates never refers to the daimonion as a daimōn; it was always an impersonal "something" or "sign".
The Hellenistic Greeks divided daemons into good and evil categories: Eudaemons (also called Kalodaemons) and Kakodaemons, respectively. Eudaemons resembled the Abrahamic idea of the guardian angel; they watched over mortals to help keep them out of trouble. (Thus eudaemonia, originally the state of having a eudaemon, came to mean "well-being" or "happiness".) A comparable Roman genius accompanied a person or protected and haunted a place (genius loci).
The notion of the daemon as a spiritual being of a lowly order that is largely evil and certainly dangerous has its origin in Plato and his pupil Xenocrates; when the later connotation is read back anachronistically into Homer, the result is distorting: "To emancipate oneself from Plato's manner of speech is no easy matter", Walter Burkert remarked. Daemons scarcely figure in Greek mythology or Greek art: like keres their felt but unseen presence was assumed. There was one exception: the "Good Daemon" Agathos Daemon, who was honored first with a libation in ceremonial wine-drinking, and especially in the sanctuary of Dionysus, and whose numinous presence was signaled in iconography by a chthonic serpent.
After the time of Plato, in the Hellenistic ruler-cult that began with Alexander himself, it was not the ruler but his guiding daemon that was venerated, for in Hellenistic times, the daimon was external to the man whom it inspired and guided, who was "possessed" by this motivating spirit. Similarly, the first-century Romans began by venerating the genius of Augustus, a distinction that blurred in time.
They are impure and wandering spirits, who, after having been steeped in earthly vices, have departed from their celestial vigour by the contagion of earth, and do not cease, when ruined themselves, to seek the ruin of others; and when degraded themselves, to infuse into others the error of their own degradation. These demons the poets also acknowledge, and Socrates declared that he was instructed and ruled at the will of a demon; and thence the Magi have a power either for mischief or for mockery, of whom, however, the chief Hostanes both says that the form of the true God cannot be seen, and declares that true angels stand round about His throne.
These spirits, therefore, are lurking under the statues and consecrated images: these inspire the breasts of their prophets with their afflatus, animate the fibres of the entrails, direct the flights of birds, rule the lots, give efficiency to oracles, are always mixing up falsehood with truth, for they are both deceived and they deceive; they disturb their life, they disquiet their slumbers; their spirits creeping also into their bodies, secretly terrify their minds, distort their limbs, break their health, excite diseases to force them to worship of themselves, so that when glutted with the steam of the altars and the piles of cattle, they may unloose what they had bound, and so appear to have effected a cure. The only remedy from them is when their own mischief ceases.
The dæmons are real enough — "the principle is the same, which misleads and deceives, and with tricks which darken the truth, leads away a credulous and foolish rabble" — it is relying upon them that is deceptive. In this way the dæmons passed easily into Christian "demons."
In the process of Christianizing Roman populations in the official Christianity from the late 4th century, theologians, hermits and monks, and the bishops and presbyters who influenced individuals, had their own repertoire of ideas, which were derived from Scripture and from the ambient culture of Late Antiquity. Within the Christian tradition, ideas of "demons" derived as much from the literature that came to be regarded as apocryphal and heretical as it did from the literature accepted as canonical.