Was Buddhism the main religion in ancient India, because ancient kings were mostly Buddhists such as the Pala kings?

No, Buddhism was not the main religion in ancient India. In fact, ancient India was predominantly Hindu, with Hinduism being the primary religion followed by the majority of the population. Buddhism emerged as a separate religious tradition in the 6th century BCE, founded by Siddhartha Gautama (Buddha). While Buddhism gained significant popularity and patronage from some rulers and regions, it never became the dominant religion in ancient India.

The Pala dynasty, which ruled parts of the Indian subcontinent from the 8th to the 12th century CE, did have a strong association with Buddhism. They were known for their patronage of Buddhist institutions, scholars, and art. However, their rule was limited to specific regions, and Hinduism remained the predominant religion in most parts of ancient India.