Anthropological research usually finds that primitive societies are polytheistic; the two normal varieties are some combination of animism and shamanism. All Eurasian societies were polytheistic before Judaism, but as we all know, the Jewish religion eventually spawned two revival cults after the fall of the Hasmonean Kingdom and eventually the Abrahamic religions took over all of Europe and Western Asia. If we view religion as a choice of belief, then we can see the world of religion as a marketplace for people to select religious beliefs from the religions they know of. Most religions purport to offer an invisible truth about the nature of imperceptible realms of existence - the origin of the world, and mankind. Even though syncretic beliefs exist increasingly in the developed world, historically religions usually proclaimed that their own view of these mysteries was the truth. Other religions could appreciate some aspects of the same truth, just in different or actually inferior terms. Since religions compete in such a way for the position of expounding the truth, with human beings acting as the arbiters or perhaps the 'consumers' of religion, it would be friendly to the logic of the major religions themselves to suppose that a religion's "success" is best measured by the number of adherents it can claim across the world. Historically, there has just been no contest in this category against the success of the two major monotheistic religions: Christianity and Islam.
World-historically, peoples that came into contact with Christianity in its European homeland were very often eager to embrace the religion for themselves. Actually, Islam is a distant result of the prestige of Christian monotheism as it reached into Arabia. However Mohammed, the prophet of Islam, envisioned the religion as a purification of the Roman monotheism and foretold that the destruction of the Roman Empire would be the eschatological destiny of the Muslims. For him and the early Muslims the Roman Empire was the dominant power of the age, and the Empire's claims to universal political and religious hegemony both meant that the true monotheism, Islam, necessarily had to overcome and destroy it as part of expounding the true religion, vindicating it. Pagan rulers from nations such as the Avars, the Bulgars, the Rus, or the Hungarians opted to switch directly to Christianity instead.
Nor did late-adherents to Christianity merely embrace the veneer of the religion for purely political purposes. Boris I, the Bulgarian Khan who brought Christianity to Bulgaria, asked for answers to over 100 doctrinal questions from Pope Nicholas I, many of which concerned questions of conscience and the real issue of what Christianity meant for Old Bulgarian customs and beliefs. The Rus, according to their own history, decided in the first place that they wanted a monotheism for the sake of its sacred powers - they then went shopping amongst Islam, Judaism, and Christianity for whichever one was best. Baptism for previously pagan leaders oftentimes meant changing their name to one from the Christian canon - as in the case of Istvan, the first King of Hungary.
What merits did all of these world-historical leaders perceive in monotheism, that they switched from their current faiths? Historically the Avars, the Bulgarians, the Slavs, the Hungarians - these leaders were well-positioned in relation to their urbanized neighbors and spent most of the Early Middle Ages as aggressors against the Byzantine Empire and Central Europe. They were not forced to convert against their will, like so many indigenous in America or the Pacific (even though there are some, like the Philippinos, who willingly converted). Is it that there is some intrinsic appeal to monotheism, perhaps particularly concerning the quest for authority, charisma, or truth? Or are the circumstances all-important?
?
