Why were Shinto and Buddhism blended in Japan for a thousand years?

Because when Buddhism arrived from China in the 6th century, the Japanese did not choose between it and their own kami worship: they assumed both were true. Over centuries the two became so intertwined that this blend, called shinbutsu-shūgō, the "syncretism of kami and buddhas", was Japan's main organized religion until the 1860s. Temples were built at shrines and shrines at temples, and the same buildings often served both.

A theory made it make sense. Called honji suijaku, it held that some kami were local "traces" of Buddhist deities, their "original ground". The sun goddess Amaterasu, for instance, was seen as a form of Dainichi, the Great Sun Buddha. A poetic phrase explained why buddhas would disguise themselves: to help beings, they had "dimmed their radiance". Later, some thinkers flipped it, making the kami the originals.

Then politics intervened. In 1868 the new Meiji government ordered kami and buddhas separated, partly to curb the wealth and power of Buddhist institutions. The order unleashed a violent anti-Buddhist movement: an estimated 40,000 temples were destroyed, with monks forced back into lay life or made Shinto priests.

Yet the separation never fully worked. Japanese families still needed Buddhist funerals and ancestral rites, and many temples still hold small Shinto shrines today. Historians even debate whether Shinto as a separate religion is ancient, or partly a product of that very separation.