“The Holy One manifests in myriad forms; I sing the glory of the forms.”
If one takes a kind of gestaltic view of non-dualism, wherein figure/form and ground form a whole gestalt, then God (or Brahman, or Tao, etc.—depending on one’s particular view) is seen as the ground of being from which, in which and of which all manifestations (figures/forms) are generated. (Protestant theologian Paul Tillich had a “pre-trinitarian” formula: ground-of-being, power-of-of being, and form-of-being [his term was “being-itself”, i.e., existentiated being]; the non-dualist Kashmiri Shaivites have a similar formulation).
We never actually perceive the ground, we only perceive the figure/forms (whether individually: the tree; or grouped: the forest). But we can only perceive any figures/forms because there is some ground against which to perceive them (I would suggest that the same is true with regard to thought-conceptualization as well). The ground is only intimated—but the figures/forms and the ground are ultimately inseparable, being necessary aspects of the whole gestalt.
The Sufi Ibn Arabi, in order to distinguish between “God” as the generative ground and “God” as encompassing the whole gestalt, applied the name Allah to the former, and the name al-Wahid (“the One” ) to the latter; both are Quranic names of God. [I doubt that I’m doing justice to Ibn Arabi with this simplification, however.]
The “One” refers, in non-dualistic thought, to the Whole, the all-in-all-without-another—the gestalt. Different religious philosophies assign different attributes to this One.
What any non-dualist system rejects is supernaturalist-dualist theism: i.e., that God is a being wholly separate from the cosmos. (Using the word “supernatural” here to mean extra-natural or non-natural.)
This kind of “gestaltic” non-dualism is neither, it seems to me, a simple “sum of parts” pantheism (which by its very terms retains a trace of dualism: separable parts to sum), nor a reductionist monism in which the figure/forms are not “really real” but delusions. That is why I (following such as the third Zen patriarch Seng Tsan and others) prefer the term non-dualism to either monism or pantheism. I am a non-dualist.
Such non-dualism is found across religious philosophies, e.g., in Buddhism, Advaita Vedanta, Kashmiri Shaivism, Sufism (in Islam), Kabbalah (in Judaism) and people like Meister Eckhart in Christianity (although supernaturalist-dualists would reject those streams of the otherwise mono-theistic religions). I am not wholly aligned with any of them, though I sometimes might call myself a Zennist, simply because I think that Zen represents the sparest expression, which I often need to return to; but I wander through all of them.