Kūkai's father, a man named Saeki-no-Atai-Tagimi, was a member of a family of hereditary regional officials in charge of local governance and religious affairs. His mother, traditionally known as Tamayori-hime, was a member of the Ato clan of educators. Her older brother served in the capital as a personal tutor to one of the sons of Emperor Kammu (r. 781 – 806).

Kūkai's family were devout Buddhists, and their family temple was dedicated to Yakushi-nyorai, the Buddha of healing. Despite his extensive writings, Kūkai did not provide any descriptions of his own childhood, so very little is actually known about this period of his life. As is often the case for important historical figures, legends have arisen to fill in the gaps in our knowledge.

One story, for example, tells how young Mao would often fashion images of the Buddha from clay. Another describes a vision in which he sat upon a lotus and engaged in conversations with various Buddhas and Bodhisattvas.

Education in the Capital

When he was 15 years old, Kūkai went to the capital in order to study at the national university. After three years of preparatory lessons from his own uncle, Kūkai enrolled in the Faculty of Letters, or Myōkyōka, as it was known. For several reasons, Kūkai's admission to the university was extraordinary. For one thing, at the age of 18, he was three to five years older than most of the other students. But more significantly, even though it was one of the most powerful members of the aristocracy, Kūkai's family had been disgraced as a result of their opposition to the decision to move the capital from Nara. It is widely believed that Kūkai's uncle was instrumental in helping his nephew matriculate in this exclusive school. Of course, Kūkai's own aptitude must certainly have played a role.

The courses at the Faculty of Letters were designed for the training of high-level bureaucrats to serve the central government. They focused on the Chinese classics, as well as courses in the spoken Chinese language. Though the prospects of serving in a government post did not ultimately appeal to Kūkai, it is believed that his education at the national university helped him develop linguistic and poetic skills that he would later use to great effect.

Discovery of Kokūzō-Gumonji

It was during this period that Kūkai made a fateful encounter, which he later described in his work The Indications of the Three Teachings (Sangō shiki). According to his own account, Kūkai met a practitioner of Buddhist austerities who acquainted Kūkai with a secret mantra and method of meditation known as the Kokūzō-Gumonji-no-hō, which is associated with the Bodhisattva Ākāśagarbha (Kokūzō, in Japanese). The man told Kūkai that anyone who performed the Kokūzō-Gumonji-no-hō with care and perseverance would attain mastery and understanding of all sacred Buddhist texts. While reciting the Ākāśagarbha, Kūkai experienced a vision in which the Bodhisattva told Kūkai to go to China to learn the secrets of Esoteric Buddhism.

Growing disappointed with his formal education at the university, Kūkai abandoned his studies and began to devote himself to the practice of Buddhist asceticism. He shaved his head and entered the priesthood at the age of 20 (though it was not until 804 that he received official ordination at Nara's Tōdaiji Temple).

In Japan at this time, Buddhism was largely a state-sponsored religion and many of its temples were under the control of the central government. Most priests were sanctioned by and received stipends from the state. Kūkai, however, became a private priest and practiced his faith without official recognition.

Life as a Mendicant Priest

Transformed by his discovery of the Kokūzō-Gumonji-no-hō, Kūkai returned to the mountains of Shikoku where he set himself to the performance of the rigorous austerities that promised to give him ultimate insight. He spent his days like a mendicant priest, subjecting himself to various forms of self-deprivation, making long forays through the mountain wilderness and engaging in extended periods of prayer and meditation.

One notable experience from this time occurred while Kūkai was reciting the Ākāśagarbha mantra. As he repeated this incantation from a cave overlooking the sea, Kūkai had a vision in which the morning star grew brighter and nearer until it finally flew into his mouth as he chanted.

The morning star, or Venus, was thought to be a manifestation of Ākāśagarbha. In other words, Kūkai had become one with this Bodhisattva. The name that Kūkai gave himself is written with the Chinese characters for sky and sea. It is thought that he may have chosen his name at this time.

In the year 797, when he was 24 years old, Kūkai wrote Indications of the Three Teachings, in which he argued for the superiority of Buddhism to both Confucianism and Taoism. Not much is known about the seven years between this work and the time Kūkai set sail for China (804).

It is believed that he spent his days studying the vast body of Buddhist scriptures. According to legend, Kūkai was not fully satisfied with the texts available in Japan at that time. He prayed fervently to have the one true teaching revealed to him.

Finally, he had a dream in which he was told about a copy of the Mahāvairocana Sutra (Dainichi-kyō, in Japanese) hidden beneath the pillars of a Kume-dera temple in Nara. Kūkai set out for this temple, and, as foretold in his dream, he found this text exactly where he was told it would be.

A pivotal chapter in the life of Kūkai began on the sixth day of the seventh month in the 23rd year of the Enryaku Era (804 AD). It was on this day that Kūkai departed for China as part of a diplomatic mission to the T'ang Dynasty.

Four ships set out from Bizen province (present-day Saga Prefecture). Kūkai, who travelled as a student monk at his own expense, was aboard the flagship, which also carried the chief envoy, Fujiwara no Kadonomaro.

Kūkai's goal in travelling to China was to learn more about the meaning of the Mahāvairocana Sutra, which he now considered to be the seminal work of Esoteric Buddhism.

He was, in short, looking for someone to teach him the meaning of this text. He appealed to Emperor Kammu and received permission to go to China with the expectation that he would remain there for 20 years.

As the Japanese ships crossed the Sea of Japan, they encountered a storm and were separated from one another. The ship carrying Kūkai and the chief envoy ended up drifting ashore at Fukien Province far to the south of their intended destination of Ningpo (formerly known as Mingzhou). Here, they encountered more trouble as the local governor was suspicious of the unexpected and uninvited foreigners. It was only after Kūkai wrote an elegant appeal in formal Chinese that the delegation was permitted to carry on.

Receiving the Teachings of Esoteric Buddhism

Kūkai made his way inland to the city of Ch'ang-an, which, at the time, was the largest, most cosmopolitan city in the world.

It was also a center of Esoteric Buddhism, which had been gaining popularity in China since its introduction to the country in the eighth century.

At the time of Kūkai's arrival, the most eminent master of Esoteric Buddhism was the priest Hui-kuo (746 – 805), the abbot of Ch'ing-lung Temple.

Unfortunately, Kūkai had to wait more than five months before finally meeting Hui-kuo. And by that time, Hui-kuo was of failing health and preparing for death.

Their encounter, nonetheless, was a joyous one as Hui-kuo instantly recognized Kūkai as the person to whom he was meant to transmit the secret teachings of Esoteric Buddhism. To the surprise, no doubt, of everyone present, Hui-kuo greeted Kūkai saying, "I have been waiting for you to come...until now, there was no one to whom I could pass along the secret teachings."

Arrangements for Kūkai's formal ordination were made, and he was initiated into the secrets of the two mandala realms of Esoteric Buddhism as well as other orders. The approaching death of Hui-kuo must certainly have created an air of urgency as the master prepared his new pupil for the transmission of the precepts and teachings of Esoteric Buddhism.

Other monks worked feverishly to make copies of sacred texts and to gather together ritual implements and Buddhist images for Kūkai to take with him back to Japan. In addition to the precepts of Esoteric Buddhism, Kūkai was also instructed in the Sanskrit language of India and received training in calligraphy and poetry from the leading teachers of the day. He was a fast learner. Hui-kuo himself famously compared the way that Kūkai mastered his lessons to the process of pouring water from one vessel into another.

By the time Kūkai set sail on his return voyage in late summer of 806, he had been initiated into the most profound secrets of Esoteric Buddhism and was now the heir to the Dharma professed by the Celestial Sun Buddha, Dainichi Nyorai. He had come to China to learn about Esoteric Buddhism. He was leaving a little more than two years later as the eighth patriarch of the sect. He was only 32 years old.

To signify Kūkai's status as heir to the Esoteric lineage, Hui-kuo bestowed on him the title Henjō-kongō, which means "The Diamond that Illuminates the Four Directions."

One famous legend concerning the origins of Mt. Kōya says that before departing from China, Kūkai threw his ritual scepter, or "vajra," into the air toward Japan with the prayer that it would guide him to the place where he was to establish his monastery.