Yes, there was. At a crude level, polytheism was the default in the Ancient World, and monotheism evolved when polytheism no longer served elite interests.

In Ancient Greece and in the republican and early imperial stages of Rome, polytheism was the default. There are several reasons for this-

• By the time a central authority emerged over Greece i.e. Alexander the Great in c.336 BC, many cities had their own gods they favoured i.e. Athens and Athena. Toleration of many gods was easier than conflict.

• This policy proved useful in the kingdoms of Alexander the Great's successors (323 BC onwards) as in many of these kingdoms i.e. Egypt, Greeks were the minority. By adopting the gods of the people they conquered, for example Isis from Egypt, or making new gods such as Serapis, a fusion of Osiris and Apris, the rulers won goodwill from the local population. When Rome annexed these kingdoms, they largely left the gods as they were.

• It allowed an imperial cult to function. Alexander had introduced a new concept to the Greeks, that of the deified ruler and this would be adopted by his successors such as the Ptolemies, and later the the Roman emperors

Now to explain the transition. From 235-284 AD, the Roman Empire went through a period of disorder, known as the crisis of the Third Century. Many emperors were assassinated, and the Romans were defeated by the Persians, with emperor Gordian III being killed. It seemed the gods had failed, and with so many emperors, imperial cults became less relevant. At the same time, monotheistic ideas were spreading. Within the Roman empire itself, Christianity which argued the old gods were not real, became increasingly popular. The early Christians shared a lot of goods in common and provided a useful support network for the poor during these times of crisis, and by arguing there was one God, they created a parallel society to the mainstream pagan world. In Persia, the Sassanians (224 AD) had adopted the old Iranian religion of Zoroastrianism as their state religion, as a way to distinguish themselves from their predecessors, the Greek influenced Parthians. Zoroastrianism was largely monotheistic.

In the Roman world, Emperor Diocletian and his eventual successor Constantine would try to adopt some aspects of these movements to strengthen the failing empire. Religion and court ceremony became more important under Diocletian. He had seen the success of the Persians in war and moved his capital east to Nicomedia to better fight the Persians- this meant more educated Romans would come into contact with Persian ideas. He adopted many of the trappings of the Persian court including requiring his subjects to prostrate themselves in front of him as if he was divine (proskynesis) and wearing a gold crown, making the emperor a semi-divine figure so he would be less likely to be the target of assassinations. Constantine permanently moved the capital east to Constantinople, and granted toleration to the previously banned Christians in 325 AD, on his death bed becoming a Christian himself. Most of the succeeding emperors were now Christians and lavished money on churches not temples. Temples fell into decay. In 363 AD, the last pagan emperor died during yet another unsuccessful Persian campaign. He had tried to de-Christianize the empire, and from this point on, the Christian emperors became increasingly hardline- consider that emperor Diocletian and his tetrarchs had persecuted the Christians from 303-313, but by 392 emperor Theodosius I had outlawed paganism, closing temples and banning sacrifice. Christianity was viewed as a unifying force across the fracturing Roman empire, with converted barbarians serving in the Roman military i.e. the half Vandal Stilicho.

To conclude, monotheistic Christianity was adopted by the elites of the Roman empire as a way to unite a warring empire against the largely monotheistic Sassanians. Outside of these two empires, which later became Christianity and Islam, polytheism still exists to this day in Hindu parts of India, Japan and much of the rest of the world.