Thomas Aquinas. Summa Contra Gentiles Book 1. trans. Anton C. Pegis. New York: Hanover House, 1955.
In God there is no passive potency. With that one sentence one can deduce most of classical theism. Deny that sentence, and one’s theology is fraught with peril, if not outright heretical. Because of that loaded sentence, one should begin the journey here and not with Summa Theologiae. The latter text is usually too difficult without a prior knowledge of medieval philosophy.
There are places where Thomas is wrong. This is not one of those places. If you have a heterodox doctrine of God (EFS, theistic personalism, etc), and when you are corrected on it, it does no good to say, “But Thomas was wrong on x, y, and z.” Maybe he was, but that is not the issue under discussion.
When Thomas Aquinas uses terms like act, potency, and motion, he does not mean by them what you mean. For example, when we say God is immobile, we mean that there is no potency in him requiring something other than God to activate God. Motion is the act the of something that is in potency. Since there is no potency in God (i.e., no unrealized aspect), then there is no motion in God.
Furthermore, God is eternal. This appears to be more familiar to today’s readers. Thomas’s reasoning will not be. When we say God is eternal, we mean God has no internal motion. If God is not eternal, then he must be brought into being by another. Again, we are back to the original statement: there is no passive potency in God. If there were, then God would depend on someone (or thing) beside himself to be God.
This eliminates any form of composition or any denial of divine simplicity. All composites have both act and potency. Moreover, composites are subsequent to components. As James Dolezal has so eloquently stated, “All that is in God is God.”
Even though Thomas has not yet said that all of God’s attributes are identical to his essence, one can see where he is going. That raises a question, though. If the divine names signify the essence, then how are the names not synonymous with one another? Aquinas answers that they do not signify the same notion.
Continuing upon this line of thought, God is his essence. The essence of a thing is either the thing itself, or it is related to it by some cause. Yet nothing can be the cause of God. Therefore, God is his essence.
Can there be two perfect Gods? No. If two Gods are equally perfect, then there must be some way to distinguish them—something must be added to one or both. But if something is added to a God, then he (or she) cannot be perfect.
Those of us in the Reformed tradition would do well to pay attention to his remarks on God’s knowledge and will. God knows all things by his essence. That is fairly standard in Western Christianity. That is God’s natural knowledge. God also knows all possibles. That is God’s knowledge of vision. And since all potencies arise from him, the First Cause, he knows an infinite array of possibilities.
Thomas’s comments on divine willing are very useful for modern discussions of free will and determinism. Given that God is his willing, and God is a necessary being, does this make everything in the world “necessary”? No. When God wills something, he wills it to the “ordered end of his goodness.” I think Thomas is arguing for something like secondary causes. He uses the example of a doctor and medicine.
On another line of thought, when God wills things, he wills things “insofar as they participate in his goodness.” Since no created thing’s participation is entire in the essence of God, there is no 1:1 willing.
In conclusion, this text is probably the second place to start one’s journey on Thomas Aquinas. On Being and Essence is the most accessible, especially the edition by Armand Maurer. The introduction should give the reader a decent grasp on the issues involved. With that under the reader’s belt, Summa Contra Gentiles should be no trouble.
 Thomas Aquinas, Summa Contra Gentiles Book 1, (New York: Hanover House, 1955), sect. 15.
 Ibid, sect. 18.
 James Dolezal, All that is in God: Evangelical Theology and the Challenge of Classical Theism, (Grand Rapids, MI: Reformation Heritage Books, 2017).
 Aquinas, Summa Contra Gentiles, sect 35.
 Ibid, sect 21.
 Ibid, sect 42.