Saichō did not live long enough to work out a thorough synthesis of esoteric Buddhism and the one-vehicle teaching of the Lotus Sūtra, and the task would be carried on by his disciples. The integration of Tendai/Lotus doctrine and the esoteric teachings (enmitsu itchi) would become a major feature distinguishing Taimitsu—the Mikkyō that developed within Tendai—from that of Tōmitsu, the Mikkyō of Kūkai’s Shingon tradition, and was essential to the development of medieval Tendai hongaku thought. (Page 21)

By the medieval period, notions of formless, originally inherent “perfect and sudden precepts” (endonkai), “Lotus one-vehicle precepts” (Hokke ichijōkai), or “unproduced diamond precepts” (musa kongō hōkai) came to supersede literal adherence to the specifics of the Fan-wang Ching precepts. These “formless readings” of the precepts put forth within the influential T’ien-t’ai school influenced other Buddhist traditions as well and have been seen by many scholars as contributing to a decline in monastic discipline in the latter Heian period. “Formless” understandings of the precepts, rooted remotely in Saichō’s advocacy of bodhisattva precept ordinations, were also linked to an important strand of early medieval Buddhist discourse, found in both Tendai and some of the new Kamakura Buddhist movements, which denies the validity of precepts in the Final Dharma age (mappō mukai) and makes liberation dependent on faith or insight, rather than on the cultivation of morality or the accumulation of merit through good deeds. (Page 19)

Shirato Waka has suggested a possible link between Saichō’s understanding of the Fan-wang precepts and the later emergence of Tendai original enlightenment thought. The Fan-wang ching describes its bodhisattva precepts as “the fundamental source of all Buddhas, the fundamental source of all bodhisattvas, the seeds of the Buddha nature. All sentient beings have the Buddha nature. All things with consciousness, form and mental activity, all sentient [beings] with mental activity, are all included within [the purview of] these Buddha-nature precepts. … The fundamental source of precepts for all sentient beings is pure in itself.” Here the bodhisattva precepts are said to be grounded in the Buddha nature. Since all beings have the Buddha nature, they incline naturally toward these precepts. Saichō further developed this argument: “These are the precepts which are [based on] the constantly abiding Buddha nature, the original source of all living beings, pure in its self-nature and unmoving like empty space. Therefore, by means of these precepts, one manifests and attains the original, inherent, constantly abiding Dharma body endowed with the thirty-two marks.” In this reading, the precepts are no longer an externally imposed set of regulations or moral guidelines, but an expression of innate Buddhahood and also the direct cause for its realization. Because the Buddha nature is innate, all people, clerics and laity alike, can readily practice the bodhisattva precepts, and by practicing these precepts, innate Buddhahood is naturally manifested. This theme is related to Saichō’s idea of the Lotus as opening the “direct path” (jikidō) to the speedy realization of Buddhahood. This view of practice (in this case, of the precepts) as simultaneously both the effect and the cause of Buddhahood would be developed in later Tendai hongaku thought. (Page 18)

In [Saichō’s] schemes of doctrinal classification, Saichō developed both exclusive and inclusive readings of the one vehicle that would be important to the development of medieval Tendai thought and practice. In his written debates with Tokuitsu, Saichō argued the superiority of the Lotus over all other teachings from a number of angles. For example, he asserted that the Lotus alone represents the standpoint of “effect,” or the Buddha’s enlightenment (kabun); other sūtras, such as the Avatarpsaka, reflect the standpoint of “cause,” or of those still in the stages of cultivation (inbun).49 He also distinguished the Lotus as the “direct path” (jikidō) or “great direct path” (daijikidō) to enlightenment, in contrast to both the “roundabout path” of the Hinayāna and the “path requiring kalpas” followed by bodhisattvas of provisional Mahāyāna. In Saichō’s view, a practitioner of the Lotus endowed with unusually keen faculties might even be able to realize Buddhahood with this very body (sokushin jōbutsu), though he confined this possibility to persons who had already achieved the first abode, or the fifth of the six stages of identity, which, according to T’ien-t’ai doctrine, comprise the Buddhist path. Practitioners of lesser faculties would be able to realize Buddhahood in the next lifetime, or in the lifetime after that. … [T]he doctrine of realizing Buddhahood with this very body, as interpreted by Saichō’s disciples, was crucial to the development of medieval Tendai original enlightenment thought. Saichō also interpreted the Lotus Sūtra as particularly suited to the time and to the capacities of the Japanese people, claims that would be further developed in the thought of Nichiren (1222-1282).