Powerful empires are more likely to lose their religion than small and oppressed communities. My first example is the Jews—they have suffered immense atrocities in the past 1100 years. But the Jews won’t give up their religion. Before the formation of Israel, their community in most areas was so small that they were like a large family. They could not conceive of leaving Judaism because that would have meant leaving behind their family members to suffer alone. The atrocities they endured made them cling to their faith tenaciously and obstinately.
Zoroastrianism, in its early form, arrived in the Middle East in the second millennium BC. By the sixth century BC, it was a much larger religion than Judaism—it was the dominant religion of the largest empire of that time, the Persian Empire, where Judaism was a minor religion. Even outside Persia—in the nations of Transoxiana and South Asia, there were millions of practitioners of Zoroastrianism. Yet in the seventh century AD, Zoroastrianism vanished almost entirely—today a few practitioners of this religion can be found in some pockets of India.
In the third century AD, Europe was being ruled by the powerful Roman Empire and the majority of the Europeans were pagans, yet by the sixth century most Europeans had converted to Christianity. The second time the Europeans have en masse given up their religion was between the eighteenth and the twentieth centuries. In this period, Europe was the great colonial empire of the world, yet most Europeans lost their faith and became atheists. In the two vassal states (culturally) of Europe, the USA and Canada, most people claim that they are atheists.
When the Ottomans captured Constantinople in 1453, the fate Orthodox Christianity was uncertain. But the religion survived in Constantinople and in certain parts of Europe. It became the dominant religion of Russia. When the Soviets took control of Russia after the 1917 revolution, they used the might of their government to force the Russian Christians to give up their religion. They failed. Millions of Russians obstinately clung to their Orthodox faith through the years of the worst excesses of Lenin and Stalin. Orthodox Christianity survived the communist rule and is now thriving in Russia.
Islam has been overthrown in just two places: Spain and Portugal (during the Reconquista). The invasion of the Middle East by the Western crusaders in the medieval period had the paradoxical effect of helping the Islamic forces to come together and prove the superiority of their religion and culture. This inspired most people in the Middle East and North Africa, and the barbarian tribes of Transoxiana and Central Asia to convert to Islam. In our time, the religious contest with geopolitical implications is between Western atheism and the Middle Eastern religion.