Christian scripts called Hinduism and Buddhism pagan. By definitions used by the papacy until the 1700s Hinduism and Buddhism were classified as pagan and are still in many places are considered pagan. The same is true with Confucianism and Taoism. However anyways the fact of the matter is that a third of the religious population of world is by Christian definition pagan and remains as such. Hinduism and Buddhism are growing in proportion every year as well in comparison to the declining Christian population. Even with strong states surrounding them, the Baltic's remained pagan until the 1300s. The same is also true for the Caucasus which whole it's ruling dynasties were Christianised, the Islamic conquerors found the population thoroughly pagan in many aspects with the sole exception being Armenia.Pretty much all of Europe converted the Middle east. African Animism has almost been completely wiped out by Islam and Christianity. And I would not call Buddhism or Hinduism pagan. Along with Confucianism and Taoism. In China and Japan they had had strong states that could stamp out such things. Even then it was a challange.
All of these places have one thing in common. Accessibility. The Baltic's and the Caucasus, important nexus's of trade and commerce remained mostly pagan for centuries before being brought to heel without a literal desert, sea and mountain range combined separating them. The Arabians have the latter advantage. And the advantage that unlike the Baltic and Caucasian pagans, their religion was consolidating every generation to the point that Rashidun and Abbasid Caliphs feared a pagan revival well into their reigns.