The world’s first experiment of mixing religion and politics happened in India, in the third century BC, when Emperor Asoka of the Maurya Empire converted to Buddhism. In that age, the Maurya Empire, with a population of around 25 million, was home to one-fourth of the world’s population. Asoka believed that Buddhism could unite all of humanity and bring everlasting peace in the world. He sent Buddhist missionaries to several neighboring countries. Buddhism is the only religion that has won millions of converts in many parts of the world without any violence.
The second experiment of mixing religion and politics happened in Europe, in the fourth century AD, when Emperor Constantine of the Roman Empire converted to Christianity, which was then a non-violent and a deeply theological religious movement. Before Constantine, Rome’s pagan rulers used to see Christianity as a fringe movement which appealed to the poor and illiterate class. Constantine’s conversion brought wealth, power, and grandeur to Christianity. However, the Christians did not develop a militaristic policy till the eleventh century (the time of the crusades).
The third experiment of mixing religion and politics happened in the Middle East, in the seventh century AD, when inspired by the teachings of Islam, the army of Arabs poured out of the Arabian desert and launched a series of military campaigns. Within two years of the death of Islam’s founder, in 632 AD, the Arabs had conquered Baghdad. They conquered Syria in 636 AD; Persia between 633 to 654 AD; the holy city of Jerusalem between 636 and 637 AD; Egypt between 639 and 646 AD; Carthage on the Northern African coast in 698 AD; and Europe’s Iberian peninsula in 711 AD.
In the case of Buddhism, the militarization of religion never happened; despite being the religion of the ruling class in several empires in India, China, Tibet, Japan, Ceylon, and other places, Buddhism remained a pacifist religion. In the case of Christianity, the militarization happened about 1000 years after founding. In the case of Islam, the militarization was instantaneous.
The criteria that I have used for selecting these three religions are:
1. The religion should be extant; it should be a major religion in the contemporary world.
2. The religion should have served as the official ideology of major empires in the past.
3. The religion should be expansionist—it should possess the zeal to convert all of humanity.