Is God a postulate?

Oppy is arguably the smartest philosophical atheist of his generation, so he's a useful foil:

Theoretical virtues:

Simplicity: If everything else is equal, we should prefer the theory that postulates fewer (and less complex) primitive entities.

It is clear that Naturalism is simpler than Theism: it postulates fewer kinds of entities…According to Theism, there are two kinds of entities–natural and supernatural-whereas according to Naturalism there is only one kind. Graham Oppy, The Best Argument Against God (Palgrave 2013), 7,19.

Several problems with that argument:

i) I'm not sure what he means by "primitive entities," but I assume he means something other things derive from, that's not derived from other things. If so, then Christian theism has just one primitive entity: God. But in that event, Christian theism meets the condition of simplicity. You can't get much simpler than only one primitive entity.

ii) What makes less complex primitive entities a theoretical virtue? A violin is simpler than a violinmaker. A toy is simpler than a toymaker. 

Perhaps Oppy is operating with the notion that complicated things are composed of parts. That complexity is reducible to simpler and ultimately simple constituents. A planetary biosphere is more complex than the early stages of the universe. A body is composed of parts, composed of molecules, composed of atoms, composed of elementary particles. That's a bottom-up model of reality. Reality constructed from the smallest or simplest building blocks.

But what about topdown models of creativity? Da Vinci's mind is more complex than his paintings. Bach's mind is more complex than his music. Dante's mind is more complex than his fiction. On that view, artifacts are simpler exemplifications of mentality. Instances of something more complex. 

Or take an abstract object like the Mandelbrot set. Infinitely complex, although it can be represented in finite instances. 

iii) I don't know what in particular he has in mind by supernatural entities. Plausible candidates include God, angels, demons, and ghosts. If so, his methodology is eccentric. The way we usually establish if something exists is not by whether that satisfies a theoretical virtue like simplicity, but whether there's any direct evidence, indirect evidence, or counterevidence. 

iv) Apropos (iii), supernatural entities aren't necessarily or even generally postulates. Although they can sometimes by invoked for their explanatory value, in many cases, people say that supernatural entities exist because they claim to experience supernatural entities. Not a postulate but a direct encounter. Not a posit but an observation. Now, Oppy can dispute the credibility of such reports, but it's a different category than a theoretical postulate. Realty is something we generally discover rather than intuit.