I think sometimes people think of the doctrine of divine simplicity as an odd artifact of a particular metaphysical view—say, Aquinas'. But that's the wrong way to think about it. Rather, as Maimonedes observed, divine simplicity is an expression of uncompromising monotheism.
For if God had parts, these parts would be in important ways divine. The first and most obvious reason, which I've discussed in at least one earlier post, is that at least some of God's parts would be uncreated. But only God is uncreated. Granted, the Platonist restricts this to claim that only God is an uncreated concrete entity. I think this restriction does compromise on monotheism, but even this restriction won't help here, since presumably God's parts, if he has any, are as concrete as God.
Second, a central theme in monotheism is that God not only is greater than everything else—some polytheists may think this to be true of their chief god—but that God exceeds everything else by, as one might say, "infinitely many orders of magnitude." But can a being that is composed of parts exceed the collection of his parts by infinitely many orders of magnitude? The whole can be greater than the parts taken together. But can it be so much greater than the parts, so much that God is God but the parts taken together do not threaten monotheism? If one responds that the sum of God's parts is just as God (as on classical mereology), and so God doesn't have to exceed the sum, then I have a different argument. Consider any one part x1 of God, and consider the collection X* of God's other parts. Then if God is the sum of his parts, he cannot exceed both x1 and X* by infinitely many orders of magnitude, since the sum of two things does not exceed both of them by infinitely many orders of magnitude (compare the arithmetical fact that a+b is no greater than twice the greater of a and b). And so at least one of x1 or X* threatens uncompromising monotheism.
Third, a being that is made of parts has some powers because of the parts. So if God were made of parts, he would have some powers because of something other than himself. But that certainly threatens monotheism.
Fourth, if God were not simple, then sometimes when we worship God, we would be worshiping him on account of some component of God. For instance, we would be worshiping God on account of his mercy, or on account of his justice, or on account of his beauty.
Now, we learned in Plato's Lysis that if we love a for the sake of b, then in an important sense what we really love is b. I propose a weaker analogue to this principle:
- If we worship x on account of y, then we are thereby worshiping y
- If God is not simple, our worship of God on account of his mercy (say) is worship of a component of God that is not God.
- Worship of anything other than God is wrong if uncompromising monotheism is true.
- It is not wrong to worship God on account of his mercy.
I think the last argument is the religiously deepest reason why uncompromising monotheism requires divine simplicity. Divine simplicity ensures that our worship of God has only God as the object of worship.