Why did Buddhism split into Theravada and Mahayana?

Buddhism didn't split in one dramatic break. It diverged slowly, over centuries, through two different processes. First, the early monastic community divided over the rules of monastic life and fine points of doctrine, as Buddhism spread across India. One of those lines of monks, the "Elders", produced Theravada, "the Doctrine of the Elders", today Buddhism's oldest existing school. Second, from around the 1st century BCE, new scriptures such as the Lotus Sutra championed a different ideal, the bodhisattva, and their followers called their path Mahayana, the "Great Vehicle".

The core difference is an ideal. Theravada honours the arhat, who reaches liberation, nirvana, by following the Buddha's path. Mahayana texts praise the bodhisattva, who vows to become a buddha in order to save all living beings, and treat seeking only one's own release as an incomplete goal. Some Mahayana writers called other paths Hinayana, the "lesser vehicle", a term now widely seen as condescending.

Yet the split was less like two rival churches than early Western scholars imagined. Chinese pilgrims such as Faxian and Xuanzang, who travelled to India, described monasteries where Mahayana and non-Mahayana monks lived side by side.

Geography finished the job. Theravada spread to Sri Lanka and Southeast Asia, and Mahayana travelled along the Silk Road to China, Korea and Japan. Today Mahayana is the larger branch.