1. If God exists, then God is metaphysically simple.
Now, the doctrine of divine simplicity implies that God’s nature is simple (is one thing, rather than a complex of things), and thus that each of the superlative attributes like omniscience or omnipotence will be identical to the divine nature (and thus, insofar as they are identical to the divine nature, identical to each other). What this means, though, is that the Theist who accepts premise 1 has a much easier time moving from some argument of Natural Theology which demonstrates that a being with some (or at least one) of the superlative attributes exists, to that the divine nature is exemplified (i.e., to that that being has all the superlative attributes).
Suppose, then, that we recognize the divine nature to involve some set of superlative attributes like omniscience, omnipotence, omnipresence, omnibenevolence, and so on, and we say that any one of these is identical to the divine nature. Then, if we can demonstrate that some being exists who clearly exemplifies any one of these properties, then that being exemplifies the divine nature, and insofar as they exemplify the divine nature they must therefore exemplify all of the superlative attributes identical therewith.
The problems with such a line of argumentation might be obvious though. First, the non-Theist is perhaps less likely to be convinced that the doctrine of divine simplicity is intelligible or correct than they are to believe that ‘God exists’ is intelligible or correct (respectively). Here, though, we don’t have to argue that God does exist or is simple, but that, as per the first premise, if God did exist then God would be simple. A second problem might be that even if one could agree that if Theism were correct, then the doctrine of divine simplicity would be correct, the non-theist may argue that there is no way of knowing that the divine nature is identical to superlative attributes which are left un-demonstrated. I suppose we would have to give an argument to think that the divine nature is identical with all of the superlative attributes, and that for any X, if X has any one superlative attribute, then X exemplifies the divine nature.
Perhaps we could give a weaker argument though. Let’s say that X1 is the proposition that some being exemplifies at least one of the superlative attributes. Let’s say that G is the proposition that some being exemplifies all of the superlative attributes. Then, clearly, P(G|X1) > P(G|~X1). Moreover, take S to be the proposition that if a being has any superlative attribute then it has all superlative attributes. P(S|X1)>P(S|~X1); note the Raven paradox. I’m not sure if that’s right, but surely the following would be right: Suppose that X2 is the proposition that some being exemplifies at least two of the superlative attributes. Then P(S|X2)>P(S|X1). In fact, technically, the observation of any being which does not have at least one of the superlative attributes while not having another increases the probability of S. Every non-God being is non-omniscient/omnipotent/etc., so that every non-God non-superlativeattribute being observed increases the probability that any omniscient/omnipresent/etc. being is God. That all X’s are G’s is supported by any observation which corroborates (or ‘increases the likelihood’) that all non-G‘s are non-X’s. Clearly, as the probability of S goes up, the probability of G goes up too, so that P(G|S&X1) >> P(G|~S&X1). Also, P(G|X2) > P(G|X1).
Thus, one might be able to argue that ‘probably’ all X’s are G’s, where an X is a being with at least one superlative attribute, and G is a being with all superlative attributes.