In our modern world, with widespread political and religious liberty, it's often assumed that conversion to a religion is fundamentally a personal and spiritual event. Throughout history, however, one of the most significant factors in religious success or failure has been the conversion of rulers. Convert a king, convert a kingdom.
Moses is an example of a visionary prophetic ruler who brings the entire Israelite nation to worship his newfound God, who seems to have been completely forgotten at the time of his Sinai theophany. The Israelite religious reforms of Hezekiah and Josiah, described in the Bible, were imposed by kings upon Israel, though with prophetic guidance. In ancient Egypt, Pharaoh Akhenaten, a monotheistic visionary, briefly succeeded in establishing quasi-monotheistic Atenism as his state religion, though it was ultimately rejected after his death.
The most famous royal convert of antiquity was Darius of Persia (who died in 486). His royal predecessors ruling the massive Persian empire had been polytheists. Darius' grandfather, however, had been converted to Zoroastrianism, and when Darius ascended the throne, he made his family religion the state religion of Persia.
The most well-known and historically significant example of this phenomenon is Constantine the Great. Before his conversion to Christianity in 312, perhaps only 10 percent of Romans were Christian. Within a century after his death, that number reached 80 percent to 90 percent. Imperial patronage was a key factor in Christianity's ultimate success.
But Constantine was hardly the only or even the first pagan king who converted his kingdom to Christianity.
Tiridates III of Armenia converted in 301. He was followed a few decades later by the conversion of Ezana of Axum (Ethiopia) around 350. Although France had been largely Christianized under the Roman Empire, the conversion of the Frankish barbarian king Clovis in 486 led to the establishment of Christianity as the religion of the kingdom of the Franks.
The Anglo-Saxons, on the other hand, remained pagans for two more centuries. When Olaf Tryggvason became king of Norway in 995, he attempted to force the Vikings to convert to Christianity, launching decades of civil war in Scandinavia. The conversion of Duke Mindaugas of Lithuania to Christianity around 1250 led to the eventual conversion of the Lithuanians, the last European people to be Christianized.
The conversion of the Slavs of Russia to Greek Orthodoxy was likewise facilitated by the Grand Prince Vladimir of Kiev. When he was baptized in 987, he ordered the entire population of Kiev to march to the river and be baptized as well. Vladimir's neighbors to the east, the Turkish Khazars living north of the Caspian, converted to Judaism following the example of their Khan, who built a replica of the Mosaic Tabernacle and renewed Jewish biblical sacrifices.
The pagan Mongol conquerors of Iran ultimately converted to Islam following the example of their Khan Ghazan, who became a Muslim in 1295. West African king Mansa Musa (1312-1337), a devout Muslim, was instrumental in spreading Islam through the empire of Mali.
Buddhist history also contains many similar examples of the conversion of kings as the catalyst for converting kingdoms. The most famous is Ashoka, the Hindu emperor of India who converted to Buddhism around 270 B.C., spreading it throughout India. Missionaries from Ashoka ultimately converted both Devanampiya Tissa, of Sri Lanka, and Menander I (Milinda, c. 145 B.C.), the Greek successor of Alexander the Great and ruler of Bactria (north Afghanistan). Afghanistan remained a majority Buddhist country for the next thousand years.
Further east, Kubilai Khan's conversion to Tibetan (Lamaistic) Buddhism made it the official religion of China in the late 13th century. Likewise, the conversion of the Mongol Altan Khan to Tibetan Buddhism in the 16th century facilitated its spread throughout Mongolia, where it's still predominant today.
The Protestant Reformation survived in the early 16th century in large part because of the conversion of German princes to Protestantism. The divide today between German-speaking Protestant and Catholic regions still reflects the patterns of which princes in the 16th century converted to Lutheranism or remained Catholic. Most notoriously, Henry the Eighth's "conversion" to Protestantism was instrumental in transforming England from Catholic to Anglican.
Throughout history, until the development of the idea of separation of church and state in the 18th century, the religion of kings has been a decisive factor in the conversions of peoples. The secular ideology and religious beliefs of rulers continue to play an important, though no longer decisive, role in societies today.