Haecceitism is the doctrine that there are haecceities. But what is an haecceity?
Suppose we take on board for the space of this post the assumptions that (i) properties are abstract objects, that (ii) they can exist unexemplified, and that (iii) they are necessary beings. We may then define the subclass of haecceity properties as follows.
A haecceity is a property H of x such that: (i) H is essential to x; (ii) nothing distinct from x exemplifies H in the actual world; (iii) nothing distinct from x exemplifies H in any metaphysically possible world.
So if there is a property of Socrates that is his haecceity, then there is a property that individuates him, and indeed individuates him across all times and worlds at which he exists: it is a property that he must have, that nothing distinct from him has, and that nothing distinct from him could have. Call this property Socrateity. Being abstract and necessary, Socrateity is obviously distinct from Socrates, who is concrete and contingent. Socrateity exists in every world, but is exemplified (instantiated) in only some worlds. What's more, Socrateity exists at every time in every world that is temporally qualified, whereas Socrates exist in only some worlds and only at some times in the worlds in which he exists.
Now suppose you are a classical theist. Must you accept haecceitism (as defined above) in virtue of being a classical theist? I answer in the negative. Franklin Mason answers in the affirmative. In a comment on an earlier post, Mason gives this intriguing argument into which I have interpolated numerals for ease of reference.
 When God created the world, he knew precisely which individuals he would get. Thus  he didn't need to have those very individuals in front of him to know which ones they were. Thus  there must be a way to individuate all possible individuals that in no way depends upon their actual existence.  Such a thing is by definition a haecceity. Thus  there are haecceities.
I don't anticipate any disagreement with Mason as to what an haecceity is. We are both operating with the Plantingian notion. We disagree, however, on (i) whether there are any haecceities and (ii) whether classical theism is committed to them. I deny both (i) and (ii). In this post I focus on (ii). In particular, I will explain why I do not find Mason's argument compelling.
My reservations concern premise . There is a sense in which it is true that when God created Socrates, he knew which individual he would get. But there is also a sense in which it is not true. So we need to make a distinction. We may suppose, given the divine omniscience, that before God created Socrates he had before his mind a completely determinate description, down to the very last detail, of the individual he was about to bring into existence. In this sense, God knew precisely which individual he would get before bringing said individual into existence. Now either this description is pure or it is impure.
A pure description is one that includes no proper names, demonstratives or other indexicals, or references to singular properties. Otherwise the description is impure. Thus 'snub-nosed, rationalist philosopher married to Xanthippe' is an impure description because it includes the proper name 'Xanthippe.' 'Snubnosed, rationalist, married philosopher,' by contrast, is pure. (And this despite the fact that 'married' is a relational predicate: necessarily, to be married is to be married to someone or other.) Pure descriptions are qualitative in that they include no references to specific individuals. Impure descriptions are nonqualitative in that they do include references to specific individuals. Thus 'person identical to Socrates' is a nonqualitative description.
Now if God has before his mind a complete pure description of the individual he wills to create then that description could apply to precisely one individual after creation without being restricted to any precise one. (Cf. Barry Miller, "Future Individuals and Haecceitism," Review of Metaphysics 45, September 1991, p. 14) This is a subtle distinction but an important one. It is possible that Socrates have an indiscernible twin. Call his 'Schmocrates.' So the complete description 'snub-nosed, rationalist philosopher, etc.' could apply to precisely one individual without applying to Socrates, the man in the actual world that we know and love as Socrates. This is because his indiscernible twin Schmocrates would satisfy it just as well as he does. The description would then apply to precisely one individual without being restricted to any precise one. So there is a clear sense, pace Mason, in which God, prior to creation, would not know which individual he would get. Prior to creation, God knows that there will be an individual satisfying a complete description. But until the individual comes into existence, he won't know which individual this will be.
As I see it, creation understood Biblically as opposed to Platonically is not the bestowal of existence upon a pre-existent, fully-formed, wholly determinate essence. It is not the actualization of a wholly determinate mere possible. There is no individual essence or haecceity prior to creation. Creation is the creation ex nihilo of a new individual. God creates out of nothing, not out of pre-existent individual essences or pre-existent mere possibles. Thus the very individuality of the individual first comes into being in the creative act. Socrates' individuality and haecceity and ipsiety do not antedate (whether temporally or logically) his actual existence.
Mason would have to be able rationally to exclude this view of creation, and this view of the relation of existence and individuality, for his argument to be compelling. As it is, he seems merely to assume that they are false.
Could God, before creation, have before his mind a complete impure description, one that made reference to the specific individual that was to result from the creative act? No, and this for the simple reason that before the creative act that individual would not exist. And therein lies the absurdity of Plantingian haecceities. The property of identity-with-Socrates is a nonqualitative haecceity that makes essential reference to Socrates. Surely it is absurd to suppose that that this 'property' exists at times and in possible worlds at which Socrates does not exist. To put it another way, it is absurd to suppose that this 'property' could antedate (whether temporally or logically) the existence of Socrates.
We are now in a position to see why Mason's argument is not compelling. If  is true, then  doesn't follow from it. And if  follows from , then  is false. Thus  conflates two distinct propositions:
1a. When God created the world, he knew precisely which pure complete descriptions would be satisfied.
1b. When God created the world, he knew precisely which individuals would exist.
(1a) is true, but it does not entail
2. God didn't need to have those very individuals in front of him to know which ones they were.
(1b) entails (2), but (1b) is false.
I conclude that classical theism does not entail haecceitism. One can be such a theist without accepting haecceities. This is a good thing since there are no haecceity properties!