Tendai Sect is recognized as exoteric Buddhism (teachings by Shaka which are made easy to understand for general people) because it has Hoke-kyo Sutra (Lotus Sutra) as its basis, or Konpon kyoten (primal scripture), but in the middle of the 9th century when Ennin and Enchin went to Tang and learned Esoteric Buddhism, the Tendai Sect in Japan also began to have the color of Esoteric Buddhism, too, and began to perform incantations of Esoteric Buddhism, and was started to be called Taimitsu.
天台宗は法華経を所依、つまり根本経典としているため、顕教（釈迦が一般にわかりやすく説いた教え）とされているが、9世紀の中ごろ円仁・円珍が唐に渡って密教を学んだことから日本における天台宗も密教的色彩を濃くし、密教で行う加持を盛んに行うようになり台密と呼ばれるようになった。