The Roman Empire was a military power but it failed to protect its pagan religion. Both halves of the Roman Empire, the Western and the Eastern, lost their pagan religion and underwent cultural transformation before and after they fell.
With the issuance of the Edict of Milan in 313 AD, Constantine the Great made Christianity legal in the Roman Empire. But this did not lead to a large-scale conversion to Christianity. Paganism continued to be the dominant religion. The number of Christians was very small. The Christian believers were sometimes prosecuted, though the official policy of the Roman Empire was to leave the Christians alone.
In 476, when the Roman Empire fell to the Visigoths, Christianity was a minority religion in Europe. The Visigoths were devout Christians and under their rule, there was large-scale conversion to Christianity. Thus, the fall of the Roman Empire led to the fall of Rome’s pagan religion and the rise of Christianity in Europe. The Roman Gods became irrelevant. The Visigoths had trounced the Romans on the political and the religious fronts.
In the eighth century, Christianity was the dominant religion in the Middle East and the Eastern Roman Empire (Byzantine Empire) was the dominant political power. The Romans could not sustain their supremacy in this region. Between eighth and the eighteenth centuries, the much of the population of the Middle East converted to Islam.