Tathāgatagarbha buddhism is deeply mystical, and the core of the Bodhisattva path of Zen (Mahayana Religionst). Many spend decades pondering the Tathagatagarbha Sutras, deeply meditating on the foundational concepts that are presented.
For many Zen seekers and aspirants, the entire Tathagatagarbha doctrine becomes too much, too religious for most to stomach. Most pop Zen books totally are absent on this subject, some only reference the Tathagatagarbha doctrine lightly and in a very basic view. Few, if any, Zen Centers even talk about this Tathagatagarbha doctrine.
The Lankavatara Sutra, is more of a commentary and analysis/explanation of the Mahaparinirvana Sutra and becomes the religious foundation of Zen/Ch’an (thus given the Sutra status). As the above links says, “Zen can not be discussed or understood without an understanding of how tathagatagarbha is taught in the Lankavatara Sutra”.
ālāyavijñāna (storehouse consciousness) is another deeply mystical concept that comes from Yogācāra – again this is a mystical “knowing” that becomes ‘Too much’ for the pedestrian Zen seeker, too religious.
Yet, for the religious-minded, then the Tathagatagarbha doctrine becomes essential to understand.
Theravada totally reject Tathagatagarbha Sutras and considered them apocryphal. Any notions of the Bodhisattva, Buddha-matrix, storehouse consciousness, Buddha-womb, and of course ‘the soul’ are rejected. Many Zen students end up reading Theravada Sutras and commentaries because they are led to believe ‘Buddhism is one big-tent’ and receive conflicting messages and views – thus quitting Zen (Mahayana) because they see Zen as nonsensical and confusing.
I have been a Zen Buddhist devotee for 35 years. I spent the first 10 years living in Zen Centers and Zen Monastic practice centers here in the USA. I found that very few have the mind or the desire to delve deep into the Tathagatagarbha doctrine, to explore and unlock the secrets. At one center, I was told that my ‘Zen mysticism’ was not welcome because it is ‘Way too religious for them.
In other Mahayana Paths, for example, the teachings of Padmasambhava is deep mysticism yet few Tibetan Buddhist practitioners really know of Padmasambhava or have read anything from this aspect of Buddhism.
Some have taken a non-religious view of Mystical Buddhism (Zen and Tibetan) and find that Science and Religion start to blur especially with Padmasambhava. Many have also taken the Tathāgatagarbha buddhism to a non-religious scientific view as well, yet find the same dilemma of the blur between science and religion, in where it becomes ‘one in the same’. If you talk to theoretical physics experts of quantum mechanics, they have often discussed that there is a mystical religious point they get to, and that shocks them!
I have been doing the online Zen since the AOL days, back in the late 80’s. There is little interest in religious mechanics with these forums. Most simply just want see Zen as a way of achieving happiness and bliss, and not really ponder much beyond that.