The Tendai teachings were brought to Japan from China by the Japanese monk Saicho in 804 C.E. Volumes could be (and have been) written on the Tendai teachings. What follows is a brief summary of some of the most important.
In China, Saicho studied the T’ien T’ai teachings formalized by the great teacher Chih’I (538-597 CE). Chih’i emphasized the importance of unifying the various Buddhist philosophies that had spread throughout China and saw the Lotus Sutra teachings as the vehicle for this focus. The Lotus Sutra is, thus, a seminal work for Tendai philosophy. One of the most important teachings, if not THE most important teaching, of the Lotus Sutra is the teaching of skillful means (upaya) – that the Buddhas teach according to the capacity of the listeners, and that the teachings thus may appear to differ, though they in reality are all just different perspectives on the One Buddha Vehicle (ekayana) to perfect enlightenment.
A second major teaching of the Tendai School is the unity of the absolute and provisional, the concept of the three truths in one truth. All things are empty, i.e. dependently arising, but at the same time, they do have a temporary existence. Therefore, all things are BOTH empty and existing. This concept is referred to in the Tendai teachings as the Truth of the Middle Way. One of the major goals of Tendai practitioners is to utilize methods promulgated by our great masters to gain experiential understanding of the Truth of the Middle Way. Shikan meditation practice is a prime example of these methods. (Shi is Japanese for samatha, i.e. calming the mind, and kan is Japanese for vipasyana or insight meditation.) Mandala practice and other esoteric practices are, likewise, commonly utilized methods.
A third important teaching is that of Ichinen Sanzen – ‘Three Thousand Realms in One Moment.’ This teaching is essential to an understanding of the role of the individual in the life of the world. We take this to mean that in our thought process there are 3,000 separate phenomenons in each instant of thought. Our thoughts radiate out from this moment affecting other sentient beings, the world and the cosmos. Thus, a thought not only affects the individual thinker but becomes part of the intricate pattern which influence, and ultimately make up, society and the universe. That thought may bring about conflict and disharmony or peace, tranquility and Buddhahood.
No less important than the others is the teaching of Light up a Corner of the World (Ichigu wo Terasu) as promulgated by Dengyo Daishi (Saicho). This is a commitment to engaged Buddhism. To assist others as oneself is to follow the teachings of Shakyamuni Buddha. Through this mechanism we give of ourselves, our time and our money for the benefit of others. This is not a mere acknowledgment of our social responsibility. It is an active pronouncement of the nature of spiritual attainment and the profound role of our worldly deeds in the manifestation of the Dharma.
Further, as a Mahayana school, the Tendai teachings incorporate the Bodhisattva ideal and precepts. By keeping these moral precepts – as either a monastic or a lay practitioner – one is able to benefit oneself and all sentient beings. Developing the aspiration for enlightenment (Boddhichitta) is a goal for all those on the Bodhisattva path.
Finally, an historical note: The Tendai school established by Saicho in Japan became a training platform for many monks who went on to form other schools of Japanese Buddhism such as the Rinzai and Soto Zen School, Jodo-shu (Pure Land), Jodo Shin-shu (New Pure Land) and Nichiren schools.