"All beings are intrinsically Buddha," as Hakuin said, means that all sentient beings are endowed with the wisdom and virtuous power of the Buddha and are, without exception, gradually advancing along the path of liberation. It is inevitable that all human beings will perfectly realize their essential nature.
Each of us, as perfect and infinite existence, is of necessity on the path to buddhahood, even when in the phenomenal world. This is what we call "the process of becoming a buddha."
To sum up this process: (1) We are intrinsically endowed with buddha nature, and although (2) we cling to the illusion of a self, (3) life continues. Also, (4) cause and effect are inevitable, (5) myriad buddhas actually exist, and (6) sentient beings mutually interact with them. It is also a real fact that (7) self and other are not two. Since all of these conditions are already perfectly met, (8) it is only a matter of time before we will, without fail, break through the deluded dream of self, awaken to our intrinsic buddha nature, and achieve wisdom of supreme enlightenment.
When one believes and understands these, one correctly believes and understands Buddhism. If we condense these further, they all boil down to the eighth tenet, that all beings are in the process of becoming buddhas. In other words, we believe that all sentient beings attain the Way of the Buddha. This is true faith in Mahayana Buddhism.
The Brahmajala Sutra says:
O all you people, believe unmistakably in your hearts that you are accomplished buddhas. "I am already an accomplished buddha." If you believe always in this way, then you are already endowed with the precepts.
Dogen Zenji says:
Buddhas and ancestors were once like we are; we shall come to be buddhas and ancestors.
Since the buddhas are people who have already accomplished the Buddha Way, they are "accomplished buddhas." And since we are already on the way to achieving the Buddha Way, we are known as "future accomplished buddhas."
The buddhas and ancestors were originally ordinary people like us, and at some point we too will definitely become buddhas and Zen ancestors. Developing faith in this is known as "developing true faith" and is called the "stage of true determination and not retreating."
To believe and practice in this way is the Zen of the Buddha Way—it is Mahayana Zen. If a person truly believes in Mahayana Buddhism, then whether they are monk, nun, or layperson, the whole of his or her life will be directed toward the ultimate objective expressed in the final verse of the bodhisattva vow: "The Way of the Buddha is unsurpassed; I vow to attain it."
At mealtime we recite the verse: "... we accept this food to achieve the Way of the Buddha," and which contains the lines,
"The first taste is to cut off all evil;
the second taste is to practice all good;
the third taste is to save all beings;
may they all attain the Way of the Buddha."
If we were to sum up the aim of Buddhism in a single phrase, it would be this: may all beings attain the Way of the Buddha. The eight great tenets are a step-by-step exposition of the contents of the Buddha Way. The Great Way, whereby we achieve the ultimate and most fundamental goal of humanity, is the Zen that has been correctly transmitted from the buddhas and Zen ancestors.
When I say that the aim of Buddhism comes down to "become Buddha," what do these words mean? The sutras tell us, "All things return to one" and "The three worlds are products of the mind alone." So we could say that "become Buddha" has its origin in oneness, or the mind alone. But what does oneness or mind alone return to? Any speculation here is futile. Recall Case 45 of the Blue Cliff Record:
A monk asked Joshu, "All things return to one. Where does the one return to?"
Joshu replied, "When I was in Seishu I had a robe; it weighed seven pounds."