Dr. Edward Feser writes a helpful reply to Dr. Roger Olson’s criticisms of classical theism. In the course of his response, Dr. Feser notes that the logic that led older theologians to classical doctrines like divine incorporeality and impassibility did so by way of divine simplicity. For some contemporary thinkers this is a point against those doctrines, as simplicity, it is argued, should be rejected. Two of the strongest facts alleged against divine simplicity are the Triunity of God and the freedom of God in creation. Christian supporters of simplicity must be honest and acknowledge the gravity of the charges here; a conflict with these doctrines would put one outside the mainstream of what has classically been called Christianity. However, it is worth noting that the arguments from these facts have not gone without reply from those who support (or are at least friendly to) it.
On the matter of the Trinity, Dr. James Anderson (a Reformed philosopher) has dealt extensively with the meaning of the classical doctrine of the Trinity, and how one can rationally believe it despite its mysterious character, in his work Paradox in Christian Theology. An extensive review of this book can be found here. Dr. Anderson’s work is pertinent to the present question because it shows the classical doctrine of the Trinity was mysterious precisely for the reason that it wanted to affirm both simplicity/unity and triplicity in God. Dr. James Dolezal (another Reformed philosopher) has written capably on the doctrine of simplicity in his work God without Parts, and in chapter 7 of this book discusses the problem of divine freedom, providing an answer that in many ways follows Dr. Anderson’s approach to mystery. Dr. Dolezal provided two comments online that helpfully elucidate his view here and here (point (4) in the latter comment is especially relevant). I will add one further comment from Dr. Thomas Weinandy’s book Does God Suffer? that I think expresses why this second problem arises for creatures thinking about God:
While we must conclude that God acts in the act of creation by no other act than the pure act that he is, we cannot conceive the nature of such an act since it is completely beyond the realm of the created order. As D. Burrel writes: ‘[I]t makes no sense to ask how pure-act acts, since it is ipso facto in act. So God’s acting involves no mechanisms, no process (from potency to act), no powers by which divinity acts.’ … G. Grisez states that the act of creation is ‘unintelligible’ in that it ‘cannot be placed in some familiar category. Creation, being unique, cannot be placed in any of the other modes of causality, and so creation is not intelligible by assimilation to anything else’… . This is an excellent example of knowing what the mystery is but being unable to comprehend the mystery. (132, n. 61)