Does the One Great Matter matter? Yes — the one great matter is the most important matter of all: The matter is the one and only matter that really matters. The matter is great.
So exactly what is the One Great Matter? Hokke-ten-hokke is the chapter of the Shobogenzo in which Dogen refers to the one great matter most frequently. Dogen says:
This is why [the Buddha] has “manifested himself in reality,” calling “sole reliance” on the “One Vehicle” “the one great matter.” Because this manifestation in reality is itself “the one great matter,” there are [the words] “buddhas alone, together with buddhas, just can perfectly realize that all dharmas are real form.” The method for that is inevitably “the One Buddha Vehicle,” and “buddhas alone” necessarily teach its “perfect realization” to “buddhas alone.” “The many buddhas” and “the Seven Buddhas” teach its “perfect realization” to each individual buddha, buddha-to-buddha, and they cause Śākyamuni Buddha to “accomplish” it.
It is “the One Buddha Vehicle” in which “sole reliance” is decisively “the one great matter.” Now it is “manifesting itself in the world.” It is manifesting itself at this place.
(Referring to the Lotus Sutra:) The fundamental point of this sutra is the purpose of [the buddhas’] appearance in the world. Although it expounds many metaphors, [the sutra] does not go beyond this. What is that purpose? Only the one great matter. The one great matter is just the Buddha’s wisdom itself; it is to disclose, to display, to realize, and to enter [the Buddha’s wisdom]. [The one great matter] is naturally the wisdom of the Buddha and someone who is equipped with the wisdom is already a buddha.
Because [the Flower of Dharma turning] is the “sole existence of the One Buddha Vehicle,” and because it is the Flower of Dharma with “form as it is,” whether it is the turner or the turned, it is “the One Buddha Vehicle,” and “the one great matter.”
When I read Hokke-ten-hokke I sense two subtlety different interpretations of what the one great matter is:
(1) The one great matter is the Buddha's wisdom and sitting meditation (zazen) is the one vehicle — the white ox cart and the best, most reliable vehicle — with which we can realize the wisdom of a Buddha. As per the preceding post, 'riding' is applying the wisdom we experience during sitting in real life.
(2) The one great matter is sitting. In this case, "perfect realization" is something we only ever experience during zazen. The only time we can be free of our own delusion and the delusion of others is during zazen.
That zazen is the Buddha's wisdom and an expression of the one great matter is definitely something I sense is true. For me, the question is whether we can only ever enter, experience and express the Buddha's wisdom by sitting.
I can find quotes and evidence in the Shobogenzo and elsewhere to support either answer to that question. At the moment, I think maybe allowing the ambiguity is more important than answering the question.
The one great matter —
True wisdom of the buddha
Realized through zazen.