Kūkai founded Shingon, Japan's tradition of Esoteric (Vajrayana) Buddhism, after bringing its teachings and ritual texts back from Tang China. In the "Jūjūshinron" (Ten Stages of the Development of Mind) he ranked religious and philosophical teachings, including Confucianism, Daoism, and other Buddhist schools, in ten ascending stages culminating in Shingon, and taught that enlightenment - becoming a Buddha in one's present body (sokushin jōbutsu) - is attainable within a single lifetime through esoteric ritual practice.
Intellectual setting
Kūkai traveled to Tang-dynasty China as part of an official Japanese embassy and returned with newly systematized esoteric teachings and texts largely unknown in Japan, founding Mount Kōya as a monastic center deliberately apart from the established Nara Buddhist schools.
Key ideas
Sokushin jōbutsu (becoming a Buddha in this very body)
Kūkai taught that enlightenment need not wait through countless cycles of rebirth but can be attained within one's present physical existence through esoteric ritual practice uniting body, speech, and mind (the "three mysteries") with the cosmic Buddha Mahāvairocana.
The ten stages of mind (Jūjūshinron)
Kūkai organized the full range of religious and philosophical teachings available in his day into a hierarchy of ten stages of spiritual development, treating each earlier stage as a genuine but partial insight superseded by the complete truth of Shingon esoteric Buddhism.
Legacy
Kūkai remains one of the most significant figures in Japanese religious and cultural history; Shingon Buddhism's ritual and philosophical system has continued for over a millennium, and he is traditionally also credited with founding Japan's first school open to commoners.