The controversial theological doctrine of Divine Simplicity holds that God is essentially a simply entity, with no metaphysical parts. It implies that God's attributes are really identical (e.g God's goodness = omniscience = eternity = necessity = etc.)
Thomistic philosopher Edward Feser defends this view in his excellent introductory book Aquinas. He says:
"For Aquinas, God is simple in the sense of being in no way composed of parts... One famous implication of this doctrine is that though we distinguish in thought between God's eternity, power, goodness, intellect, will and so forth, in God himself there is no disctintion between any of the divine attirbutes. God's eternity is his power, which is his goodness, which is his intellect, which is his will and so on. Indeed, God himself just is his power, his goodmess, and so on, just as he just is his existence, and just is his essence" (p.126-127)
The obvious and most common objection to the implication mentioned by Feser is that God's attributes are different, not identical. For example, by "eternal" we mean that God has not beginning nor end. By omniscience, we mean that God knows every and only true propositions, etc.
But Feser has two replies to this objection. He comments:
"we can ackowledge that the expresions "the morning star" and the "evening star" differ in sense while consistently affirming that they refer to one and the same thing (the planet Venus), so too can we acknowledge the obvious fact that "power", "goodness", "intellect" and so on differ in sense while insisting that when applied to God they refer to the one and the same thing" (p.129)
This first reply is based on the concepts of sense and reference, and how many concepts with different senses have the same referent.
William Lane Craig, a critic of Divine Simplicity, rejects the above reply and says "being the morning star and the evening star are distinct properties, both possesed by Venus; the same entity has these two distinct properties. In the same way, being omnipotent and being good are not two different senses for the same property (as are, say, being even and being divisible by two) but are clearly two distinct properties" (Creation Out of Nothing, p.178)
I agree with Craig. I see no compelling reason to think that omniscience (for example) is the same property than "eternity", but with a different label. They seem to be clearly two different properties, not the same property. The example of the "morning star" and the "evening star", while referring to the same thing (Venus), also refer to different properties of that same thing. In other words, they describe two different properties of the same thing.
Likewise, omnipotence and eternity, while being attributes of the same God, refers to distinct properties of God (note that a property, like eternity, can be predicated of things other than God, like numbers and other abtsract objects for example. Therefore, prima facie, it seems impossible that eternity, which can be predicated of non-divine objects, be identical to omnipotence which only can be predicated of God. This simple fact suffices to think that eternity and omnipotence are two different properties).
Feser also have a second reply to the objection based on Aquinas' doctrine of analogy: "While the terms we apply to created things do not apply to God in either equivocal or univocal sense, they do apply in analogical senses. So, while it of course would be absurd to say that power, goodness, intellect, and so forth are identical to God if we were using these terms in exactly the same sense in which we apply them to ourselves, it is not absurd to say that there is in God something that is analogous to power, something analogous to goodness, something analogous to intellect, and so on, and that these "somethings" all turn out to be one and the same thing." (p.128)
Again, I find Feser's reply unconvincing. First, Feser says that the terms that we apply to ourselves cannot apply to God in any equivocal or univocal sense. Fine. But what about terms that ONLY apply to God like omnipotence and omniscience? These terms clearly don't apply to "created things", let alone to finite and imperfect human beings. These terms apply exclusively to God.
And in this case, again, each term seems to refer to distinct properties, not to just one. Omniscience is the property of knowing all and only true propositions. And omnipotence is the power of bringing about everything which is logically possible. Clearly, we're talking of two different properties here, and the distinction between them have nothing to do with how we apply these terms to created things (e.g. human beings) since these specific terms DO NOT apply to them at all (only apply to God).
On the other hand, that there is something "analogous" to power, intellect, eternity, etc. which turn out to be one and the same thing only can be accepted if we have an account and definition of exactly what that "something" is. As far I know, no clear definition of that "analogous something" (which is identical to all the divine attributes) has been provided. How could be "eternity" defined analogously in a way that make it identical to omnipotence or to aseity?
In conclusion, and being largely an "outsider" in theological questions, I side provisionally with Craig in his rejection of the doctrine of Divine Simplicity.
In any case, I fully recommend Feser's books (including
Aquinas), and a careful reading and study of his
blog.
You'll learn a lot of good, erudite and sophisticated philosophy in Feser's works.