How did Buddhism and Hinduism spread most successfully

Buddhism and Hinduism which had a great influence on the Southeast Asia civilization, spread more successfully when Indian merchants as well as Brahmans and Buddhists monks settled down throughout this region bringing with them their religious beliefs around 530 BC. Besides, when those merchants travelled along the silk roads, they shared their faith with other societies contribuiting to the spread of these two major religions.

Buddhism and Hinduism are two religions parallel in some aspects to each other but divergent in theory and practice in many other cases. The Vedic religions, Buddhism and Yainism, share a regional culture located around the Indian northeast, which today is Uttar Pradesh, Bijar and Nepal. It was in this region that the Brijad-araniaka-upanishad, considered among the oldest Upanishads, was compiled under the rule of King Yanaka of Mithila. Both Siddhartha Gautama (later known as Buddha) and Majavira (the historical founder of Yainism) come from this region.

Ancient India had two lines of thought: the Shramana and Vedic religions (today continued in Hinduism). Both Buddhism and Yainism are continuation of the Shramanic traditions that have coexisted with the Vedic tradition. Both currents have existed together for thousands of years, influencing and influencing each other, during all that time.

Ancient Buddhism was also heavily influenced by Brahminical cosmology, incorporating some of the concepts that appear in the Upanishads.

Siddhartha Gautama accepted and incorporated into his doctrine several existing beliefs, but bearing in mind that to achieve "enlightenment" should not be accepted as faith nor the authorities, nor the scriptures nor the existence of a god.

In turn, Indian religions later appeared, were influenced by new interpretations and new ideas of Buddhism.

Buddhism had a certain predominance in the Indian subcontinent, but was eclipsed in its place of origin by Hinduism and Islamism. Instead, Buddhism flourished outside of India. Thus Tibetan Buddhism can be seen as a continuation of Buddhism as it existed in India before the Muslim invasions. Today it predominates in the Himalayan region, as Theravada Buddhism (in Sri Lanka) and as Zen Buddhism (in East Asia).