It is within one's own heart that one must seek Enlightenment and Omniscience. Why is this? Because the heart is naturally perfectly pure.
This school of esoteric Japanese Buddhism is the outgrowth of the religious and philosophical studies of a precocious youngster. A young man named Kukai was already studying Taoism and Confucianism at the capital at the age of fifteen. Soon, though, he would become attracted to Buddhism, and go on not only to found an influential school of thought within that expansive religion, but to become one of the most popular of Japan's own homegrown saints. An excellent biography of Kukai, or Kobo Daishi as he became known after his death, can be found here; this account will instead briefly explain the basics of Shingon.
Kukai formulated his teachings and theories in a work known as Juju Shinron, roughly, "The Ten Stages of Development of the Human Consciousness". Kukai's idea of Buddhism focused mainly on one of the later sutras: the Mahavairocana. The Buddha revealing the Doctrine in this case, Vairocana, was the "Great Illuminator" and one of the five Mahayana Jinas. According to Vairocana, the heart was the bearer of the natural purity which makes each of us a Buddha. The highest goal of one seeking enlightenment is thus to "purify the heart and to become conscious". Of course, purification of the heart could only be achieved in tandem with purification of body, word, and thought, which were in turn attained by practice.
Shingon identified Vairocana for the first time with the abstract entity known as Dharmakaya - the Ultimate Reality. The object of Shingon is the ultimate realization that one's nature is identical to Vairocana (as, according to Shingon teachings, the historical Buddha was simply one manifestation of Vairocana). The doctrines of Shingon are passed down orally from master to student, and are believed to issue directly from Vairocana.
Shingon is essentially a Tantric, or Vajrayana, school: it promotes the attainment of Buddhist enlightenment through the seemingly "worldly" process of physical action and interaction with objects (for those with "superior faculties" who will not be lured into samsara). The purification of the body is achieved through mudra, or devotional gestures, oftentimes accompanied by ritual instruments. The mind is enlightened through meditation, which in Shingon temples used as their focus specific mandala. These two sacred images were diagrams of Vairocana's twin aspects: kongo-kai, the Diamond World, and taizo-kai, the Womb World. The growth of this sect's popularity within the Japanese aristocracy, based partially in Kukai's own prestigious reputation, produced temples of amazing splendor and mandala of the highest detail. Speech was purified by the student's dedicated recitation of mantra associated with the sect. Simplified Shingon mantra, as well as mudra, became well-known charms and wards against evil entities and misfortune.
Shingon first achieved its wide-ranging cultural influence during the Heian period, in which it became one of the principal schools of Japanese Buddhism, with more popular support than the influential Tendai school. Buddhism's friction with Japan's local, traditional religion, Shinto, was addressed directly by the development of Ryobu Shinto ("dual Shinto") during the Kamakura period. Great shrine-temple complexes outlined the identification of Buddhas and bodhisattvas with Shinto kami, and Buddhist priests presented offerings for these composite beings - promoting harmony much in the manner of Greek dominance around the Mediterranean and the days of Amun-Zeus. Vairocana, for instance, was held identical to the Shinto sun goddess Amaterasu. While Ryobu Shinto, like most schools of Japan's traditional beliefs, lives on mainly in its influence on secular culture and holidays, Shingon remains a living, active part of Japanese Buddhism, with an estimated 12 million adherents today.
Brosse, Jacques. Les maitres spirituels. Trans. Sara Newbery; English text edition W & R Chambers Ltd., New York, 1991.
and a great deal of help from one amazing hand.