There are reasons to believe that they were:
Let's put monotheistic religion as Christianity, since it is the most widespread in our days.
The post-diluvian peoples gradually separated themselves from that primitive tradition and from the idea of a single god, ruler and governor of all. And they were gradually contaminated with diverse beliefs, human inventions and perhaps actions of the devil, of that first idea of a single transcendent god.
With time, as they united, fought and conquered each other, the beliefs were logically merging and mixing with each other and little by little they were leaving aside that primordial truth that the ancestors had narrated.
In the Greco-Roman world, so closely linked to each other, they understood that, although there were many deities, there was a celestial hierarchy. There was a supreme divinity who ruled over the rest of the gods, who, in turn, were followed by other lower divinities or intermediate spiritual beings, which they called "daimon" (spiritual beings who were in the lives of humans for good or evil).
For example, in the "Apology of Socrates", a text of Plato where he narrates the death of Socrates, he says that he never had divine interventions of god who spoke to him, but he had a daimon (a spirit), who was guiding him in a negative way, that is, he did not tell him what he had to do, but he did tell him what he did NOT have to do.
Socrates was a martyr of the Old Testament of the truth of one god, of monotheism. Socrates said "there is only one god". That is why he is accused of being an "atheist" and was tried and killed for it.
What did Socrates and the Greeks in general say? He said that Zeus (or Jupiter) was the father of the gods, so when Cicero was going to be killed, he said the following phrase: "Pater hominumque deorum miserere mei" (Father of men and gods, have mercy on me).
Cicero did not pray to all the gods, he prayed to the father of all gods.
The famous Walter F. Otto, a German philologist known for his work on the importance and significance of ancient Greek mythology and religion, claimed that the Greeks were monotheistic, but that they personified the various divine attributes in lower divinities (that is why all Greek "gods" were anthropomorphic).
This, however, is not accepted by everyone. For example, St. Augustine and Dante said that the mythological gods were simply angels. St. Paul, on the other hand, said they were demons.
One of the keys to understand that the Greco-Roman culture was monotheistic is anthropomorphism.
Mount Olympus is a well-ordered world. The head is Zeus, son of Cronus. He is the ruler of heaven who directs all the gods and does it in a harmonious and non-violent way.
The word "Zeus" is the word that in Latin later became the word "Deus" (God), (the genitive of Zeus is Deus).
Zeus has the same root as "Theós", which in Greek means "God", "the divine", "the luminous", "the bright", "the shining", "the resplendent". And Zeus was transferred to the Latin world with the word Jupiter, that is, "father of the divine light". "Ju" is equivalent to "Zeus" contracted in Latin. Therefore the fundamental attribute of Zeus/Jupiter, is the lightning, the light, the light bearer.
Both Zeus/Jupiter and Yahweh, because of this fulminating luminous power, cannot show themselves directly to men as they are. He showed himself in the form of Athena, Apollo, Dionysus, etc. That is why he always transformed himself into animals or other men.
The divinities in the classical world are always represented with proportion and beauty. Because divinity implies beauty. That is why centaurs, minotaurs and other non-human creatures were deformed, aberrant. Half man - half bull, half horse - half man, etc.
The truth is that the Greco-Latin world and the Christian world are strictly related.
For example, do you know the myth of Pandora? When Jupiter made man, the other gods, jealous, wanted to contribute something. For this, joining efforts and gifts (the word "doron" in Greek, means gift and "dora" is the plural, "Gifts"), they created a woman calling her Pandora ("pan" means "all" and "Pandora" means "All gifts").
Jupiter, to punish the pride of those who had wanted to emulate him, gave Pandora a box containing all the evils of the world, so that she could give it to her husband on the condition that it was not to be opened.
However, they opened it and when they opened it, all the evils spread throughout the earth.
What does all this sound like to you? Genesis. The Greeks and Romans narrated the genesis long before the arrival of Christianity.
In the Greco-Roman world, as in the Christian world, all guilt must be paid for. There can be no evil without guilt. Just as there can be no good without glory.
Zeus/Jupier considered that someone had to pay for the crime committed by men towards the gods. But how does a man pay a debt to a god? A man cannot be equated with a god. What can a man give to a god that the gods do not have?
The solution, for classical Greco-Roman antiquity, was to be found in the word of a "mediator." Only a mediator, someone who mediated between men and god, could pay for that initial fault.
But in order to pay for that initial fault against the gods, that mediator could not be a simple man, because his debt was unpayable. Nor could he be a god, because the god was precisely the offended one. The answer to the solution was to be found in the dual-natured hero. The mediator had to be someone who was both man and god at the same time (known as a "hero").
What does this sound like to you? The Christian confession (the most famous monotheistic religion).
