Theos, Deus, Zeus

One root: the shining sky

From the Indo-European root *dyeu-, 'to shine; the bright sky', flowed two great families of divine words. The first is the name of the Sky Father himself, *dyēus, which became the Greek Zeus — whose oblique stem still shows the older form, in the genitive Diós ('of Zeus') and dative Dií — and the Latin Jupiter and Diēspiter.

The second is the derived adjective *deiwós, 'celestial, of the bright sky', which came to mean simply 'a god'. This is the source of the ordinary word for 'god' across most of the family: Latin deus (and dīvus, 'divine'), Sanskrit devá, Lithuanian diẽvas, Old Irish día, Welsh duw. Through Latin deus and dīvus it gives English 'deity', 'divine', and 'divinity', and through Latin's daughters the Spanish Dios, the Italian Dio, the French Dieu.

(A strange fate befell the word in Iran: the Avestan daēva, exact cognate of the Sanskrit devá, was branded by Zarathustra a name for the demons — the old gods cast down as devils, a reversal that uncannily prefigures what would one day befall the gods of Greece and Rome.)

The odd one out: theos

Here lies a famous trap. The everyday Greek word for god, theós, looks for all the world like a cousin of the Latin deus — the same meaning, almost the same sound. Yet the two are not related at all; their likeness is sheer coincidence. Theós does not descend from *dyeu-. The true Greek reflex of *deiwós survives instead in the adjective dîos, 'divine, godlike' (as in Homer's dîos Akhilleús, 'godlike Achilles') — and it is dîos, not theós, that is the real cousin of deus and devá.

The word theós itself is of disputed origin, but is most often traced to a wholly different root — *dʰh₁s-, bearing on the sacred and the cultic, the same root thought to give the Latin fēriae ('holy days'), fānum ('temple'), and fēstus ('festal'). Where deus names the god as a being of the bright sky, theós would name the god by way of the holy — as the object of worship and rite. So Greek kept the inherited Sky Father as a proper name, Zeus, yet drew its general word for 'god' from the vocabulary of the sacred, much as the Egyptians did with neter.

Plato among the etymologists

The ancients delighted to ask whence the names of the gods had come. In the Cratylus, Plato has Socrates suggest that the first Greeks, like many barbarians, held only the sun, moon, stars, and sky to be gods; and seeing them forever running (theîn) across the heavens, called them theoí, 'the runners'. He unfolds Zeus's twin names Zēna and Día, too, as a single hidden phrase: the god 'through whom (di' hon) all living things have life (zēn)'.

These are playful guesses, not sound philology — theós has nothing to do with running, and the Zeus etymology is a learned pun. Yet they are among the earliest serious attempts to read theology out of the very names of the gods, and the conviction behind them — that the divine names conceal divine truth — would run on through the Neoplatonists and into the Hermetic reverence for the sacred names of God.