Mahajanapadas

By 500 BC, sixteen monarchies and 'republics' known as the Mahajanapadas stretched across the Indo-Gangetic plains from modern-day Afghanistan to Bangladesh. The largest of these nations were Magadha, Kosala, Kuru and Gandhara. The right of a king to his throne, no matter how it was gained, was usually legitimized through religious right and genealogies concocted by priests who ascribed divine origins to the rulers.
Hindu rituals at that time were complicated and conducted by the priestly class. It is thought that the Upanishads, late Vedic texts dealing mainly with incipient philosophy, were first composed early in this period. The educated speech at that time was Sanskrit, while the dialects of the general population of northern India were referred to as Prakrits. In 537 BC, Gautama Buddha gained enlightenment and founded Buddhism, which was initially intended as a supplement to the existing Vedic dharma. Around the same time period, in mid-6th century BC, Mahavira founded Jainism.
Both religions had a simple doctrine, and were preached in Prakrit, which helped it gain acceptance amongst the masses. While the geographic impact of Jainism was limited, Buddhist nuns and monks eventually spread the teachings of Buddha to Central Asia, East Asia, Tibet, Sri Lanka and South East Asia.