Esoteric Buddhism includes central doctrines which cannot be written down and must be passed directly from teacher to student. Its rituals rely heavily on mantras (sacred sounds), mudras (sacred gestures) and mandalas (sacred images.)
Kuukai so impressed his teachers that by 806 they encouraged him to return to Japan to propagate Esoteric Buddhism there. Shortly thereafter the Chinese government began a campaign of persecution of foreign religions. Esoteric Buddhism was largely wiped out in China. Today, aside from Japan, it is mostly confined to Tibet.
Returning to Japan he gained fame not only as a great religious teacher but also as a talented civil engineer and a brilliant calligrapher. He built numerous temples, including a large complex of temples and monasteries on Mt. Koya (Kouya-san). He also set up a school that was open to all social classes.
Koubou-daishi is revered even by Buddhists of rival sects, and his life is surrounded by numerous legends. He is widely credited with inventing the Hiragana syllabary which made it possible to accurately represent the Japanese language using Chinese characters, though there is little solid evidence for this.
Shingon was never the largest sect in Japan and today it is relatively small with only about 2 million members, though it controls some of the country’s most historic and beautiful temples.