In the Medieval period most thinkers conceived of relations as properties inhering substances, rather than properties shared by two substances. As Jeffrey Brower explains:
“According to this Categories model, whenever two (or more) substances are related, this is to be explained by certain monadic properties or accidents inhering in the relata. Thus, if Socrates is similar to Theaetetus (i.e., resembles him with respect to some quality), this is not to be explained by an entity to which Socrates and Theaetetus are somehow jointly attached (namely, the dyadic or two-place property, being-similar-to). On the contrary, it is to be explained by a pair of accidents, one of which inheres in Socrates and relates him to Theaetetus, and the other of which inheres in Theaetetus and relates him to Socrates.”
Thus, for London to be north of Paris, or for it to be east of Bristol, would mean something like that London has the property of being north of Paris, and Paris would have the property of being south of London, and as London has the property of being east of Bristol, so Bristol would have the property of being west of London. For two things to be related, therefore, means for each one to bear a predicate relating it to the other. One can see why, with this idea of relations, one would avoid saying that God is related to the world, and prefer to say that even if the world is related to God (has predicates relating it to God) God is not related to the world (there are no properties inhering God which are about the world).
Suppose we thought of properties differently though. For instance, suppose that we thought of properties represented by prepositions as being dyadic – then it wouldn’t pose a problem for the doctrine of divine simplicity for God to be related to the world, so long as he created some dyadic properties which he shares with the world but which don’t strictly ‘inhere’ either God or the world. The trouble is that this is incoherent, there cannot exist any properties which do not inhere some substance (a property which isn’t a property of something). Thus, if relations are properties, and if properties always inhere one subject at a time (are monadic) then to say that God is really related to the world is to challenge the doctrine of divine simplicity according to which God is not comprised of any parts, is entirely simple, is therefore entirely immutable, and therefore cannot himself change in any respect, but for God to create a world to which he is related (rather than simply letting it be related to him) would mean for God to add to himself.
This is why God cannot be in time, and why, I think, the A-theory of time must be wrong.
Note: “G.E. Moore discusses the idealistic doctrine of internal relations; he concludes that it is false or confused or perhaps both. What is presently interesting is that he takes this doctrine to be the claim that all relational properties are internal—which claim, he thinks, is just the proposition that every object has each of its relational properties essentially in the above sense.” (Alvin Plantinga, Naming and Necessity, Page 12)
Clearly, though, a thing could have relational properties by accident, rather than by essence or nature. God, for instance, has subsistent relations (i.e., God is related to himself in a way analogous to the way I am related to me as myself; and these relations are the three persons), but God, according to the doctrine of divine/metaphysical simplicity has no other relations, no relations which aren’t subsistent or ‘in him by nature’, and the only ones he has by nature relate him to himself, and they do so in precisely suchwise as to be three subsistent persons. The relations inhere the divine nature qua the divine nature.
This increases the plausibility that a person is essentially related to at least one other person, so that in no logically possible world does there exist only one person (at all times, throughout all the states of affairs which obtain at that world – and the only reason there can be a world in which there is only one contingent person is that that contingent person would be related to God). If persons are essentially related, then the Unitarian thesis (that God is only one person, and not three persons) is incoherent.
However, isn’t to say that a person is essentially related something like saying that a person has subsistent relations? If so, what could these subsistent relations be? Maybe relations to God? An easier solution presents itself, however, if we simply think that a person has some relations essentially in the same way a mathematician has some properties essentially, like being intelligent. However, Tom the Mathematician is only accidentally a mathematician, and thus only accidentally intelligent (Tom could cease being a mathematician without ceasing to be identical to himself). That solution is short sighted though, since Tom seems to essentially be a person, but maybe the key is that Tom only accidentally exists at all. Moreover, we can say that a person X exists if and only if X bears at least one relation inherently which relates X to some other person. It doesn’t have to be a particular other person, it just has to be some other person. If this is the right way to think, then the Divine nature can be construed not as involving ‘this’ or ‘that’ subsistent relation in particular (as then the Father and the Son would both need to be identical with the same subsistent relation of standing in the paternity relation, and then they would be identical, ergo etc.) but that the Divine Nature must be construed as involving a particular combination of subsistent relations essentially, rather than essentially involving this subsistent relation AND that subsistent relation AND that subsistent relation.