The connection between the expression of the inner and the outer in Zen gardens and Christian practice is perhaps far from obvious, until one understands perspectives on body and emotion in the two practices.
In the case of Christian practice, the wafer represents the body — a necessary and vital physical connection with God — and the blood represents the emotions, Christ's passion, that is, a feeling connection to one's being. So one might say that Holy Communion recognizes—as does Zen—that what is lacking in human beings is a physical and a feeling understanding of the world. These are considered to be divine qualities in Christianity; and although Zen and Buddhism eschew divine references, the understanding for the need of balanced experience, which they refer to as enlightenment, are not in essence different from the Christian understanding.
There is an irony in this, of course; in our rush to discover a new way of feeling, we fall into a reactionary and habitual type of feeling when we see the garden.
The same pitfall awaits the Christian who attaches themselves to the form of the religion — the ritual elegance and artistic expression accompanying the practice. The lesson is the same for both practices: form begins as a path towards balanced being, and ends as a distraction from it. Only the physical anchor of the monastery and the Dharma Hall can pull us back from this habitual reaction into a contemplative mode, where we recognize the need for emptiness.
This prepares us to receive the very mystical and remarkable message conveyed by an openness to feeling, uncontaminated by the habits and opinions of our ordinary selves.