Well, Prof. Liebegott and others claim that only abstract objects can be necessary. Concrete objects cannot exist by necessity. All concrete objects must be contingent. Now, if God exists, then God is a concrete thing he is minimally a conscious mind of some sort. So God is not an abstract object like the number 1. Thus, God is not the kind of thing that could exist by necessity on this view. The claim that God is necessary is as mistaken as the claim that the number 1 itself weighs eight pounds. Nonsense.
Why believe this? Two arguments have been proposed in the previous comments. I do not think that either is compelling, but let’s look at each and then I will point out my worries.
Argument One: It is theoretically simpler to believe that only abstract objects are necessary. if one denies this claim, then one has two kinds of necessary entities: concrete and abstract. It would be theoretically simpler to only have one kind. We need necessary abstract entities, so let’s keep them. Thus, it seems likely that only these kinds of entities can be necessary.
Argument Two: Claiming that God alone is the only necessary being is ad hoc. There is no real motivation for claiming that God is different from all other concrete entities. We ought to treat all concrete objects the same in terms of their modal status.
Let me address each argument in turn. Concerning the Argument from Theoretical Simplicity let me accept that having only one kind of necessary being is preferable if possible. But if simplicity is the goal, then here is a simpler metaphysics yet: there are only concrete objects. One of them is metaphysically necessary (God) and the abstract objects are simply ideas in God’s mind (or follow from his nature or something like that.) This seems to be (roughly) Augustine’s view. It is simpler theoretically because I only need one kind of entity (concrete) and then I claim that one of these entities has a special modal status (it is necessary.)
This gets me to argument two, the Ad Hoc Argument. Why treat God as special? I think that this is not particularly ad hoc because if God exists, then he is surely unique. If God exists, then everything else that exists depends upon Him for its existence. This gives God a unique statues among all existing things. I think that it is legitimate to claim that there is a major metaphysical difference between the creator of everything (other than itself) and the created. This does not seem ad hoc. Adding an additional claim about modality does not seem arbitrary given the huge difference between God and everything else. So this does not appear to me to be a case of special pleading, but of recognizing a real difference.
Technical Note: Perhaps there may be a way to modify the objection by claiming that there is no such thing as de re modality (modal statuses of things) only de dicto (modal propositions.) I am not entirely sure what I think about this, but it doesn’t seem particularly promising to me unless one couples it with the claim that no propositions expressing existence can be necessary. But then I see no reason to believe that. Furthermore, it is unclear to me what would ground the modal status of propositions without de re modality (unless one thinks that all necessary propositions are analytic.)