Depending on the reader's preparation, "Summa Contra Gentiles: God" is either completely impenetrable or one of the easier philosophical works to understand.
While it is ideal for the reader to have read Aristotle, particularly his "Physics" and "Metaphysics", less - even much less - will do. What is minimally necessary is an understanding of the vocabulary. Thomas used a number of terms that he acquired from Aristotle that had a particular technical meaning, a meaning that is different from the ordinary meaning of those same terms. Without a good grasp of these terms, the reader simply will not be able to follow Thomas's logic.
Fortunately, the list of important terms is not very large, nor are the meanings especially obscure. The purpose of this review is to list and define these terms. For examples, I will draw on the familiar story of "The Three Little Pigs":
Matter, material - what a thing is made of. The matter of the three little pigs' houses are straw, sticks, and bricks respectively. Contrast with "form".
Form, formal - how a thing is ordered or arranged. The form of all of the three little pigs' houses is the same: "house". Contrast with "matter".
Prime matter - the stuff out of which all physical objects are ultimately made. While the third little pig's house has the form of "house" and the matter of "bricks", "bricks" themselves have a form of "brick" and matter of "earth" (assuming they are earthen bricks), and "earth" itself has a form and matter, and so on. Eventually, this process must end with some matter that is not composed of anything more fundamental. This most fundamental matter is given the name "prime matter".
Substance, substantial - Ordinarily, matter and form together make a substance. The third little pig's house is a substance that combines the matter of "bricks" and the form "house". The possibility of substances which do not ultimately derive from prime matter is an important question (perhaps the important question) of Summa Contra Gentiles.
Sensible - that which is seen, heard, smelt, touched, or tasted. Sometimes this term is used to refer to the sensible qualities themselves (color, sound, etc.) and sometimes to the objects that have those qualities. The little pigs' houses can be seen, so those houses are sensible objects. Contrast with "intelligible".
Intelligible - that which is understood but not sensed. We understand "house", but we cannot see "house", although we can see the three little pig's individual houses. Contrast with "sensible".
Accident, accidental - the qualities of a thing that do not determine what it is. The matter of which the three little pigs' houses are made is accidental; whether a house is made of straw, sticks, or bricks, it is still a house. Contrast with "essence".
Essence, essential - the qualities of a thing that make it what it is. That the three little pigs' houses are places for them to live is essential to those houses; if they couldn't live in them, they wouldn't be houses. Contrast with "accidental".
Quiddity - see "essence".
Privation - a lack of a quality that would ordinarily be present. It would be a privation if the first little pig could not see, but it is not a privation that his house cannot see.
Act, actual - what a thing is at a particular time. After the first little pig builds his house (but before the wolf blows it down) it is a house in act. Contrast with "potency".
Potency, potential - what a thing could be, but is not. Before the first little pig builds his house, the straw of which it is to be made is a house in potency. Contrast with "act".
Motion - sometimes refers specifically to movement in space, at other times to any change in a thing.
Generation - the process of applying form to matter to make a substance. While the first little pig is building his house from straw, the house is in generation. Contrast with "corruption".
Corruption - the process by which matter loses its form and ceases to be a substance. While the wolf is blowing down the first little pig's house, the house is in corruption. Contrast with "generation".
Nature, natural - qualities of a thing or changes to a thing that arise from what it is. It was natural for the sticks of the second little pig's house to stay where he put them. Contrast with "violent".
Violent, violence - motion in a thing that is contrary to its nature. When the sticks in the second little pig's house were blown apart, that change was violent. Contrast with "nature".
Eternal, eternity - often used to mean not dependent on time; as distinct from an infinite amount of time. "House" is eternal but the pig's individual houses were not.
Cause - how a thing came to be. The efficient cause of the first little pig's house was his work in building it. While Aristotle defined four causes: material, efficient, formal, and final, Thomas almost always means the efficient cause when he refers to a thing's cause.
End - why a thing came to be. The final cause, or end, of the little pigs' houses were to give them shelter.
"Summa Contra Gentiles: Book One: God" is St. Thomas Aquinas' work in which he proclaims his philosophy of God. While differing from his Summa Theologica in form, it does bear it some resemblance. It consists of 102 chapters, each of which postulates a particular attribute of God. Each chapter then proves the postulated attribute by the application of philosophical reasoning. Support of authority, Scriptural or otherwise,. is only invoked after the issue has been established.
This is a book which makes the reader think. Some chapters really leave the reader with the feeling of understanding something new. This book is not light reading. It requires the investment of serious intellectual energy. For the reader willing to make the investment, the rewards can be heavenly.