The man we know as Lucius Apuleius wrote the only entire Latin novel to survive the fall of the ancient word, The Golden Ass also known as the Metamorphoses (the Satyricon of Petronius has come to us with sections missing). It is perhaps the first picaresque novel, its hero suffering through all kinds of discomforts at the hands of many, both as a human and while transformed into an ass.
At the end of the novel, however, there is a seemingly very serious and interesting description of ceremonial in the worship of Isis and her consort, Osiris, as that goddess was worshipped in Roman times. Apuleius is initiated into the mysteries of Isis, the Queen of Heaven, Regina Caeli. Also included in the novel is a description of ecstatic dancing and blood-letting (flagellation) by eunuchs in the worship of what is referred to as "the Syrian goddess." I assume that is Cybele, given the role played by eunuchs in her worship.
Older Catholics like me will find the title Regina Caeli familiar as one given to the Virgin Mary, which could be heard in a hymn to her sung back in the old days of the Latin mass and Novenas. As for flagellation, and even ecstatic dancing, Christianity has had its proponents of both. These are reminders of the debt (if one can call it that) owed by Christianity to the ancient pagan past.
But there is one sense at least in which Christianity differs from the religion of the ancient pagans (amusingly called Gentiles by Christians). It isn't monotheism either, assuming "monotheism" is properly used to describe the three gods in one said to be the Christian God. There were monotheists among the pagans, or at least those who could claim to be monotheists as appropriately as could Christians of the time. It is instead Christianity's pretension to being the only true religion, the one path to heaven, the sole truth; that is to say its exclusivity, and its often violent intolerance of that which is not Christian.
Christianity isn't alone in this remarkable conceit of course. So are the other major religions said to derive from Abraham, Judaism and Islam, which have been equally exclusive, equally intolerant. Their gods are angry, jealous gods; we are sinners in their hands, though some sinners are worse than others. I know little of the religions of the East, but it seems to me that they have at least been less intolerant than those of the West.
The pagan religions from which Christianity borrowed so much had no such pretensions in the sense that they were generally not exclusive and not intolerant. One could, and did, worship many gods. One could be admitted to the mysteries of Isis and those of Mithras. Admission to the Eleusinian mysteries was open to all, after they ceased being purely local; several emperors were initiates.
Religion was essential to Greco-Roman society; there were temples everywhere, rituals were to be performed with great frequency. The worship of some gods or goddesses was considered silly or barbaric or contrary to social order from time to time. Christianity was indeed thought to be anti-social and was persecuted because its followers refused to participate in the ceremonies believed to be essential to the welfare of the Roman state and its people. However there was no insistence that only one god should be worshipped, no claims that worshipping other gods was evil, no crusades as in Christianity, no religious conquests as took place as was the case in Islam and Christianity, no Inquisitions.
It's interesting to consider just how it came about that the Abrahamic religions became exclusive and intolerant. If it wasn't the influence of pagan religion, what could it be? Judaism was certainly exclusive and intolerant, and there was a long tradition that Jews alone were the favorites of the one God. There were occassions as in Alexandria where fights would break out between Jews and pagans--riots, really. There was frequent tension, but nothing quite like the virtual warfare with pagans and heretics resulting from the triumph of Christianity and later Islam. Perhaps the Jews' belief in their own sanctity and righteousness didn't result in the extermination of other religions and beliefs and those who followed or held them on a larger scale because they never obtained political and military power as did Christianity and Islam.
It's likely that such intolerance derives from the belief not just in a single God but in a single truth, a single way of living that is the only way. We see the same intolerance in Nazism, fascism, Stalinism, Maoism. This intolerance seems to be associated with a figure divine or semi-divine who issues commandments to be followed or interprets those given by some other figure.
Sheep and Shepard seems an all too accurate analogy for humans and our relationship to power of all kinds. But it's curious that this kind of religious absolutism didn't develop during the time of polytheism. There was political absolutism enough in the Roman Empire, of course, but the emperors didn't begin to demand the closing of the temples and the schools of philosophy and the acceptance of a particular version of Christianity until the reign of Theodosius (he also banned the Olympic games).
An all-powerful, all-knowing god who is jealous, demanding and angry. What does this reveal about those who worship such a being and were induced to do so at the end of antiquity, and the state of the culture and society which saw the triumph of such a religion?