Universalizing and Ethnic Religions

The split comes down to who counts as a potential believer. A universalizing religion, like Christianity, Islam, or Buddhism, claims a message meant for all of humanity, so missionaries carry it across cultures and it spreads through expansion diffusion, jumping from person to person and place to place while the hearth itself keeps believing. An ethnic religion, like Hinduism or Judaism, is bound up with a specific people, their history, and often a specific land, so it spreads mostly through relocation diffusion: believers move, and their religion moves with them, but they rarely go looking for converts.

Trace the three big universalizing religions outward and the mechanism shows up clearly. Christianity started in the eastern Mediterranean and spread along Roman roads and trade routes, then through colonization, until it dominates Europe, the Americas, and parts of sub-Saharan Africa. Islam spread from Mecca and Medina through trade across the Arabian Peninsula, North Africa, and into South and Southeast Asia. Buddhism began in northern India, then rode trade routes like the Silk Road north and east into Tibet, China, Japan, and Southeast Asia through missionary monks and merchant contact, gradually fading in the India of its birth while taking root almost everywhere else.

When a universalizing religion splits into branches, the branches often diffuse on different paths and settle into different regions, so the world map shows Catholic Latin America next to Protestant North America, or Sunni Southeast Asia next to Shia Iran. That branching pattern is a direct extension of expansion diffusion working region by region. It connects back to the diffusion types from earlier in the unit: universalizing religions lean on hierarchical and contagious expansion diffusion, ethnic religions lean on relocation diffusion, and that one structural difference explains why some faith maps sprawl across continents while others stay tied to a homeland.