That Chapter 10 as a whole pertains to this principle - that's where the teaching on teachers of the Lotus Sutra being the Buddha's envoy is found (BDK translation):
"After my parinirvana, if there are any sons and daughters of a virtuous family who expound even a single line of the Lotus Sutra in private to even a single person, they should be acknowledged as the ambassadors of the Tathagata. They have been dispatched by the Tathagata and carry out the Tathagata's work. As for those who extensively teach among the common people, know that they are yet greater ambassadors."
"Medicine King, if there are good men and good women who, after the Thus Come One has entered extinction, wish to expound this Lotus Sutra for the four kinds of believers, how should they expound it? These good men and good women should enter the Thus Come One's room put on the Thus Come One's robe, sit in the Thus Come One's seat, and then for the sake of the four kinds of believers broadly expound this sutra.
"The 'Thus Come One's room' is the state of mind that shows great pity and compassion toward all living beings. The Thus Come One's robe is the mind that is gentle and forbearing. The 'Thus Come One's seat is the emptiness of all phenomena. One should seat oneself comfortably therein and after that, with a mind never lazy or remiss, should for the sake of the bodhisattvas and the four kinds of believers broadly expound this Lotus Sutra.
from: http://nichiren.info/buddhism/lotussutr ... hap10.html
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Different interpretations of this passage are possible. One conventional one has it that if one is teaching the Lotus Sutra in just this way, one is indeed embodying the body, speech, and mind of Buddha Shakyamuni. What do you call a being who is sitting in the seat of the Tathagata, wearing the robe of the Tathagata, and articulating the teaching of the Tathagata?
Also from Chapter 10:
"If there are any sons or daughters of a virtuous family who preserve, recite, explain, and copy even a single line of the Lotus Sutra, or who pay homage to this sutra with various offerings of flowers, perfumes, necklaces, scented powders and ointments, burning incense, canopies, flags, banners, clothing, or music, or who honor it with their palms pressed together, such people should be respected by the entire world. They should be revered in the same way as the Tathagata is revered. Know that these people are great bodhisattvas who are to attain highest, complete enlightenment."
I think a common feature of the Buddhist path is to always look on one's teacher as though they are the Buddha. It doesn't matter that they don't perfectly fulfill that role. There is a story in the Lotus Sutra about the Buddha's relationship with Devadatta in a previous life where the Buddha was a mere seeker and Devadatta was a brahmin who claimed to know the Lotus Sutra. He promised that if the seeker served him, he would teach the sutra. The seeker faithfully and energetically served the brahmin, but the brahmin never followed through, and instead took advantage. Nonetheless, the seeker was able to build merit dedicated to the Lotus Sutra and so was later able to encounter it. The point is not whether the teacher is perfect. *I'm going to go out on a limb and suggest Siddhartha looks perfect(ish) to us now - (he actually seems remarkably human in the Tipitaka) - only because there were generations of pious followers rounding out the rough spots and losing unflattering details to time...
Its really difficult to talk about Tientai Lotus thought without taking into account its full scheme. In the perfect teaching, cause and effect, beginning and end, while not discarded, are undermined in their literal sense. The Buddha's Original Awakening is not without cause, but is also infinitely in the past - like the end of a rainbow, forever out of reach. The same with our enlightenment - we as bodhisattvas are vowed to postpone our full enlightenment until such time as all beings are saved; if beings are infinite, then this goal is likewise infinitely far off in the future. This then in a roundabout way directs our attention on this moment, and all the dharmas play out in this moment, and this moment only. That's not novel - I think this principle itself is standard Mahayana, if not Buddhism in general, although I think this explanation is novel. Everything is then focused on the dynamics of now - what is a Buddha? What is an ordinary mortal? nominal selves are apparitions - its all circumstance and interactive dynamics. The Buddha then in a sense is the ideal embodiment of the tendency/draw toward awakening that permeates everywhere - the universal buddhanature. This could appear as a Buddha proper with 32 major characteristics. It could also appear as a father putting on rags to work alongside his long lost son; it could also appear as a Brahmin who promises to deliver a sermon on the Lotus Sutra but never does - anything that draws beings on the path to awakening. Why not a practitioner who enters the room of the Tathagata, puts on the Tathagata's robes and takes the Tathagata's seat?
We enter the room, put on the robes, and take the seat. We do our best, because if not us, then who else can take action when the beings are suffering? Do we stand around and worry that we will only lead beings to another burning house and do nothing? Or do we try, however imperfect our effort might be because really, the choice is everyone perishing fretting that we didn't train enough yet or we at least die trying to get out. Maybe some are successful.