Ryōbu Shinto is a syncretic blend of Shinto and Buddhism that originated in Japan during the Heian period (794-1185). It was developed by the Tendai school of Buddhism, which was founded by the Japanese monk Saichō in the early 9th century.

The term "ryōbu" means "two aspects" or "two teachings" and refers to the two main branches of Buddhism - Mahayana and Vajrayana. These two branches were believed to represent the two aspects of reality - the ultimate truth and the relative truth. The Tendai school sought to reconcile these two aspects through the fusion of Shinto and Buddhism.

The origins of Ryōbu Shinto can be traced back to the teachings of the Chinese Buddhist monk and scholar, Zhiyi (538-597). Zhiyi's teachings emphasized the concept of "three truths" - the ultimate truth, the relative truth, and the middle way that reconciles the two. This concept greatly influenced Saichō and his followers in the Tendai school.

Saichō believed that Shinto and Buddhism were not contradictory, but rather complementary. He saw Shinto as a form of "natural Buddhism" that could be integrated with the teachings of Mahayana and Vajrayana Buddhism. Saichō and his followers also believed that the kami (deities) of Shinto were manifestations of the Buddha, and therefore, should be revered and worshipped alongside Buddhist deities.

The Tendai school also incorporated elements of esoteric Buddhism, also known as Vajrayana, into Ryōbu Shinto. This included practices such as mantras, mudras (hand gestures), and mandalas (sacred diagrams). These practices were believed to have the power to transform the practitioner's mind and lead them to enlightenment.

Ryōbu Shinto also incorporated Shinto rituals and festivals into its practices, such as the annual purification ritual of harae. This further solidified the integration of Shinto and Buddhism in the Tendai school's teachings.

Over time, Ryōbu Shinto became increasingly popular among the aristocracy and the imperial court, and it also spread to the common people. Its influence can still be seen in Japan today, particularly in the blending of Shinto and Buddhist elements in various religious practices and rituals.

In conclusion, Ryōbu Shinto originated from the teachings of the Tendai school of Buddhism, which sought to reconcile the two aspects of reality through the fusion of Shinto and Buddhism. It was heavily influenced by the concepts of Zhiyi and incorporated elements of esoteric Buddhism and Shinto rituals. Its popularity and influence have endured throughout Japanese history and continue to be seen in modern-day Japan.