Richard E. Hennessey coins the useful term 'theomonism' to describe the onto-theological position of Seyyed Hossein Nasr. "Theomonism is the conjoint thesis that (1) there is but one and only one being, and thus the 'monism,' and (2) God is that being, and thus the 'theo.'" So there is exactly one being, and that being is God.
One wonders what creation could be on such a scheme. If God is the sole reality, and if, as is obvious, God is not a creature, then it would seem to follow that there are no creatures. Moreover, if it is necessarily the case both that God is the sole reality and that God is not a creature, then it would seem to follow that it is impossible that there be any creatures. How can it be true both that God is the sole reality and that God created the world? Hennessey quotes Nasr:
Since the One God is Infinite and Absolute as well as the Infinitely Good, He could not but create. His Infinitude implies that he contains within Himself all possibilities, including that of negating Himself, and this possibility had to be realized in the form of creation.
Hennessey glosses the quotation as follows:
There seems, that is, to be at work here a thought sequence something like the following: The creation of the non-divine is the negation of the divine. Now the divine is the real and thus the negation of the divine is the negation of the real. But the negation of the real is the creation of the non-real. It follows, therefore, that the creation of the non-divine is the creation of the non-real.
Only those among [us] who think that the many extended changing beings surrounding us are genuinely real could object.
Well, it seems to me that one could reasonably object to Nasr's theomonism even if the plural world revealed to the senses is not taken to be genuinely real. But it depends on what is meant by 'genuinely real.'
There is a clear sense in which the plural world is genuinely real: it is not nothing. Anyone who asserts that the plural world of planets and people, cabbages and computers, is literally nothing is either a fool or a sophist or doesn't understand the English language. A second sense in which the plural world is genuinely real is that it is not an illusion. This is not perfectly obvious and so requires a bit of arguing, but for now I take it as given that the world revealed by the outer senses (and their instrumental extensions) is not illusory. It may be Erscheinung in Kant's sense, but it cannot be Schein in his sense. (One could perhaps mount a Contrast Argument: Soviel Schein, soviel Sein! to invoke a German proverb.)
So the plural world is not nothing, and it is not illusory. But I would maintain that no one who holds that the plural world is a created world can maintain that the members of the plural world are independently real. So if 'genuinely real' means 'independently real,' then I would deny that "the many extended changing beings surrounding us" are genuinely real. They are not genuinely real because they are not independently real. They lack plenary reality. They are real all right; but dependently so. Assuming creatio continuans, the denizens of the mundus sensibilis are dependent at every instant on divine support for their very existence. That, I would urge, is an entailment of a sophisticated theism.
One could put the point by saying that God and creatures enjoy different modes of Being, but both truly are: creatures are not nothing and they are not illusory. This leads us back to the modes-of-Being problematic about which I have written a number of posts. (See Existence category.)
Nasr's theomonism is untenable because it denies a plain fact, namely, that there is a plural world. That is a datum, a starting point, a fact that is surely more evident than the existence of God. Extreme monism, a species of which is Nasr's theomonism, cannot accommodate the fact of plurality. A tenable theism is a moderate monism according to which there is exactly one independently real being that serves as the ultimate ontological ground of the plurality of dependently real beings.