Recently, a "certified" Soto Zen teacher told me that according to Dogen, "practice is enlightenment itself." I suggested that if one were to "qualify" that statement by expanding it to include, "enlightenment is practice," it might be an acceptable position.
I said that in my experience, whenever Dogen could be read as saying "practice is enlightenment," he qualified it by implying the "nondual" nature (which does not mean "synonomous" or "equivalent"). The Heart Sutra would be meaningless if it stated, "form is emptiness" without the inclusion of "emptiness is form." Yet, the teacher would not accept that.
Although he refused to give me any kind of explanation that might lead one to that notion, he may be right (although I don't think so).
I may be wrong (as I often am) although I do offer a partial explanation of my understanding, and how I came to it:
For Dogen, enlightenment without practice is not authentic enlightenment (and vice versa). True enlightenment is activated only coincident with true practice, and true practice is itself the activation of enlightenment.
This has always been the truth in Buddhism, and its teaching has always been susceptible to misunderstanding. The confusion between "sudden" (or, original enlightenment) and "gradual" (or, acquired enlightenment) has been the most visible and persistent manifestation of this argument in the Zen tradition.
His biographers tell us that for Dogen, it was the apparent contradiction of this doctrine that became the barrier to, and eventually the catalyst of his own great experience of enlightenment. It was through his personal resolution of the seeming contradiction between the doctrine of "original" enlightenment and the need for spiritual "practice" that allowed him to, in his words, "complete the task of a lifetime."
The term "non-dual" means, "empty of duality", it does not mean that practice and enlightenment are "one." Practice and enlightenment in Buddhism (hence Dogen) are two aspects of one reality.
The very first paragraph of the very first teaching he is purported to have written on his return from China is constructed of four variations of the question that drove him throughout his quest.
"Now, when we research it, the truth originally is all around: why should we rely upon practice and experience? The real vehicle exists naturally: why should we put forth great effort? Furthermore, the whole body far transcends dust and dirt: who could believe in the means of sweeping and polishing? In general, we do not stray from the right state: of what use, then, are the tip-toes of training?"
Fukan-Zazengi, Gudo Nishijima & Mike Cross, Appendix, v1, Shobogenzo, p.279
This is not simply a series of rhetorical statements, but an expression of spiritual realization, which calls us to a deep contemplation, much like koan-introspection. Keeping in mind his creative use of interogatives, Dogen is not neccesarily saying, "the truth is all around: we do not need to rely upon practice, put forth great effort, etc." Rather, he may be saying (and I think he is saying), "the truth is all around: why do we need to practice, who could believe in the means, of what use, etc.?" Hence, it is not rhetorical, nor is it a "question" which requires an "answer" in the conventional sense. It is the revelation of a spiritual truth.
As professor Hee-Jin Kim points out in his landmark book, "a characteristic of Dogen’s thought was that he used a number of interrogatives in the Sung colloquial language in order to express his profound metaphysical ideas such as shimo or somo (what, how, why) and other related expressions...
...[T]hese interrogatives, along with the idea of emptiness and nonthinkning, are significant in indicating that zazen for Dogen was ultimately the expression of an eternal quest for the meaning of existence, which was paradoxically enough, meaningless—it was living the meaning of ultimate meaninglessness. This is Zen.
Hee-Jin Kim, Eihei Dogen Mystical Realist, p.63 (see also; p.134-140)
Dogen realized and taught that enlightenment is the actualization of authentic practice, practice is the actualization of authentic enlightenment.
The duality of practice and enlightenment is transcended not annihilated. Dogen frequently and explicitly included the standard Mahayana teaching on nonduality for this dynamic when he used the term "Zazen" to indicate the non-dual actualization of practice and enlightenment.