Among the major world religions, Buddhism has the distinction of being the only one capable of migrating from its foreign land and blending in with indigenous beliefs. It began in India, yet the country largely practices Hinduism. It migrated to China and Japan, where it diffused with earlier religions like Taoism, Confucianism, and Shinto. Only in southeast Asia does it seem to have completely uprooted native beliefs.
Other religions are incomparable. Hinduism never made out of India. The three monotheistic giants all maintain a strong presence in their homeland, despite the diaspora of the Jews, the holy mandates of the Muslims, and the gospels of Christ which entice one to spread the faith. Though these religions have scattered widely, they usually maintain a powerful influence in any territory that accepts them, completely eradicating the previous faith, such as Zoroastrianism in Persia, or any of a number of pagan beliefs in the New World and beyond. Only does Judaism seem to diffuse the way Buddhism does, but it has never managed to claim the majority of a population.
While Christian and Islamic strangleholds can easily be explained by political congruence, Hinduism can't. There was abject stubbornness in India to cling to its original mythology, which is unique among religions itself. Buddhism though has such widespread appeal that its diffusion becomes inevitable in places where politics and religion aren't so intimately tied with each other. Furthermore, Buddhism's toleration of other beliefs has done well for its survival in distant lands. Thus it is the only truly migrating world religion.
