Kuan-Ting (also known as Chang-an) was the 2nd patriarch of the T’ien-t’ai school, although some sources cite him as the 5th. In his introduction of the Mo Ho Chih Kuan (“Great Stopping and Seeing”), the monumental work compiled from the teachings of the de facto founder of the T’ien-t’ai sect, Chih-i, he says,
What is Perfect Faith? It is the conviction that all entities are empty, that they are nevertheless provisionally existent, and that they are the middle between these extremes. Though ultimately there are not three separate views, provisionally there are three. To say separately they do not exist forestalls the interpretation that there are three, while to say there are three illuminates the truth in each of them. yet in the absence of either forestalling or illuminating the difference between them, one has conviction that all entities are alike, ultimate, pure and unimpeded. When hearing of the profundity and the vastness, not to fear or doubt; and when hearing of the shallow and the narrow, to still have courage in one’s mind – this is what is called having perfect faith.”
In the text of the MHCK itself, Chih-i says,
It is like talking about burning a candle: it is not beginning, yet not apart from the beginning, not final, yet not apart from ending. If knowledge and faith are complete, when one hears that a single instant is it [bodhicitta: the thought of awakening], by virtue of faith one does not repudiate it, and by virtue of knowledge one does not fear it. beginning and end are both right, both it.
If one has no faith, one will elevate it to the sphere of sages and think one has no knowledge of it. If one has no knowledge, one will become conceited and think one is equal to Buddha. Then beginning and end are both wrong, both not it.
In one of the footnotes of Neal Donner’s translation of the MHCK, he quotes from the Kogi, a Japanese commentary on the MHCK by Chiku (1780-1862):
Faith means to accept the teaching directly without superimposing one’s personal opinions.”
And, of course, Seng-ts’an in his poem Xinxin Ming (“Verses on Faith in Mind”) wrote,
To understand the mystery of this One-essence
is to be released from all entanglements.
When all things are seen equally
the timeless Self-essence is reached.
No comparisons or analogies are possible
in this causeless, relationless state.
Consider motion in stillness
and stillness in motion;
both movement and stillness disappear.
When such dualities cease to exist
Oneness itself cannot exist.
To this ultimate finality
no law or description applies.
For the unified mind in accord with the Way
all self-centered striving ceases.
Doubts and irresolutions vanish
and life in true faith is possible.”
Kuan-Ting translation by Neal Donner; Chih-i translation by Thomas Cleary