The fourth question in the Summa Theologica answers the perfections of God. You will find it helpful to remember the qinque viae (English, “five ways”) from Question 2, specifically the degrees of perfection from which we know God exists.
As Christians we often are seen as arrogant for claiming our God is perfect, but as you will see, being “perfect” is a requirement to be God at all, not an opinion. I don’t usually include them, but here you will see the objections that St. Aquinas answers. It’s important in some of the Questions because we have to know first of all what the antagonist would say, then St. Thomas’ response. Without further delay…
Article 1 consists of whether God is perfect. The objections state that God is not perfect because, (1) things that are perfect are “completely made” and since God is not made at all, he is not perfect, and (2) also that since God is the beginning and beginning seem imperfect, God is imperfect. Since God is pure act, he is perfection. With God there is not a need to compare “beginnings and development” or degrees of perfection occurring over time. God is simultaneously the end as well; He is outside time completely. God is not just a simple beginning but is the end as well, and thus is simultaneously the beginning and end and is perfect in that. Therefore there is no being made process or perfection over time, there just is completeness always present. This is perfection.
Article 2 consists of whether of the perfections of all things are in God. The objections are that, (1) God is simple, and the diversity of the perfections in the world cannot exist in God who is simple, and (2) that since many things are perfect would be opposite and opposites cannot coexist, this negates the ability of perfections to exist in God. The reply to these is that we have already determined that God has no accidentals, so each of these perfections are not in God as accidents because God is His very essence or nature (He is what He is). However, since God is pure act, all of these perfections are his essence and by this we mean to say that his perfections are emanating, that his essence is always acting.
Article 3 consists of whether any creature can be like God. Thomas defends that the comparison of man to God in likeness is that of analogy. Where man can be like God in likeness, but not in genus and genre, such as that of a statue being made like a human, but is not really inversely comparable. A reflection on this in my own words comes from the creation of all things by God, which are an effect. If God caused all things in creation and we are to assume that a cause is “like” its effect then we have an insurmountable amount of things that are “like” God and cannot then tell what likeness is since we have a wide range of incomparable things. The key is participation in this likeness. We are participating in his essence, being, and substance.
What’s the take away? It’s all about Article 1, that God is perfect because He cannot achieve a better version of Himself – He just is. Things in the world can achieve perfections but these things cannot be themselves perfect. God can, and God is, because God is outside space, time, and so is and always was. Say it any which way you want to – God cannot achieve anything because He already has. He’s pure perfection.
So when you hear “perfect” in reference to God, know how it is properly used. It’s not simply what we like most or which is better according to an individual or a civilization, but is the achievement of what potential a thing is capable. For God, He has no potential because He is just Himself.
Want more on Question 4? Go here for the Summa itself.