Elements of the Hellenic Cosmogony can be found in Argonautica by Orpheas, in Argonautica by Apollonius Rodius as well as in the Orphic Hymns about which Plato said that they are "the creature of a genius and not just anybody's mind" (Ogygia Vol 3, page 359). Apart from Orpheas, Hesiod is said to have written Cosmogony-Theogony years later after Orpheas.
In Argonautica by Apoll.Rhod. (Book A 496-498) among other information on the structure of the Universe we read: "Orpheas was recounting that in the beginning the Sea, the Heaven and the Earth, all three of them, were united and inseparable until they came apart because of the destructive 'quarrel' they had had." This means that the chemical elements of matter by which the Sea, the Heaven and the Earth were made up later, did not exist separately but all together as a whole mass since they had not separated from each other yet. This resulted in the domination of Night which always existed - i.e. it was there before the inseparable chemical elements of matter - as well as to the prevalence of Chaos with its unspecified powers.
The darkness of night and part of the unspecified powers of chaos that might have been the compactness and pressure, gave birth to Eros (Desire, Love) i.e. the power of Attraction and Repulsion, the marvellous power which urges into union and creation; it was then that the Universe started to get its primitive shape. That great power (Eros) caused, on the one hand, the separation of those elements which until then constituted an inseparable mass, and on the other hand, it caused the mutual attraction of chaotic elements thus resulting in the creation of the units (atoms) of matter.
In Argonautica by Orpheas (lines 423-427) we read: "I (Orpheas) first hymned Chaos - how it caused the Universe to change and how the Sky was formed; I hymned the creation of the Earth and the bottoms of the Sea; I also hymned everything that had been created or separated from each other by the oldest, perfect and most inventive power of Eros."
Furthermore, in the hymn to "Eros", Orpheas adds:
In the hymn to "Protogonus" (Eros) and in "Argonautica" by Orpheas (lines 14-17), Orpheas imagines the chaotic mass being like an egg which bursts open and there comes Eros out of the eggshell illuminating everything; that is why everything created by Eros was called "Fanis" (because when Eros first appeared everything appeared, too). This prehistoric theory is similar to the current scientific theory called "Big Bang" according to which the Universe was created in an unfathomably large explosion which occurred about 15 billion years ago. At the moment of origin all the elements of matter in the Universe existed together until the Universe exploded into the Big Bang and everything started moving away from each other.
According to Orpheas, Zeus - the God, the Spirit - swallowed "Fanis" (primitive matter contained in the "Egg") and under his own power and influence the inert matter was activated culminating in the formation of the Universe. Zeus' swallowing of "Fanis" symbolizes the fact that without the interference of the Superior Power - Zeus, God, The One - the inert matter (i.e. what was created by Eros) would be impossible to be made active.
Zeus here symbolizes the Superior Power, The God, The Creator. Their belief in One God is also clear in the hymn to Zeus were Orpheas says that He (Zeus) created everything and He is the Beginning and the End of everything and also that He is self-formed (Auto-poiesis principle) and Father of gods and humans. As for the other gods mentioned in the Orphic Hymns they are the same Superior Power merely symbolizing His different attributes and capacities.
On the evidence so far, we could infer that the initiated into Orphism believed that matter was not the only power within the Universe and without Zeus' interference it would be dead. Would it be an exaggeration, then, if we said that they, first of all, had laid the foundations of Monotheism in prehistoric times?