Paraphrasing the Summa: First part, Question 3, article 2

Article 2. Whether God is composed of matter and form?

This question is very much related to Article 1, whether God has a body.  The answer to that question is, "no, God does not have a body."  A body is composed of matter and form.  God does not have a body, because God is not composed of matter and form.

Objection 1. It seems that God is composed of matter and form. For whatever has a soul is composed of matter and form; since the soul is the form of the body. But Scripture attributes a soul to God; for it is mentioned in Hebrews (Hebrews 10:38), where God says: "But My just man liveth by faith; but if he withdraw himself, he shall not please My soul." Therefore God is composed of matter and form.

The first objection uses Scripture the same way that the last series of objections did in Article 1.  This one says that Scripture says that God has a soul.  A soul is defined as the combination of spirit and body.  Therefore, since Scripture is the Word of God, God must have a body attached to His soul.  And if God has a body, He has matter and form.

Objection 2. Further, anger, joy and the like are passions of the composite. But these are attributed to God in Scripture: "The Lord was exceeding angry with His people" (Psalm 105:40). Therefore God is composed of matter and form.

Again, Scripture is blamed for depicting God as angry and passionate.  Since these are attributes of men, then this objection concludes that God must have matter and form.

Objection 3. Further, matter is the principle of individualization. But God seems to be individual, for He cannot be predicated of many. Therefore He is composed of matter and form.

Again, bodies are composed of matter and form and are the reason why men are individuals.  Since God is referred to as an Individual, then He must have a body composed of matter and form.

On the contrary, Whatever is composed of matter and form is a body; for dimensive quantity is the first property of matter. But God is not a body as proved in the preceding Article; therefore He is not composed of matter and form.

St. Thomas says that anything composed of matter and form, is a body.   And in Article 1, we proved that God is not a body.

I answer that, It is impossible that matter should exist in God.
First, because matter is in potentiality. But we have shown (I:2:3) that God is pure act, without any potentiality. Hence it is impossible that God should be composed of matter and form.

In Article 1, replies to objections 2 and 3, we showed that God is pure action.  He is never at rest.  And only bodies can be at rest.  Since bodies are composed of substance and shape.  And God can't have a body, then God is not made of substance and shape.

Secondly, because everything composed of matter and form owes its perfection and goodness to its form;

*This doesn't make sense, unless we look at an example.  Let's talk about a particular type of matter and the many uses which it can be "formed" into.

Clay can be formed into cups, jugs, pots, bowls, dishes, ash trays, etc.  So, clay is the matter.  The form is the shape of the matter, so that would be either the form of a cup, pot, etc.  And the goodness (i.e. what its good for) of the form is the use to which it is put.  Therefore, goodness of a cup is as a container of liquid for drinking.  The goodness of a dish is a container of food for eating.

St. Thomas, then, is saying that the use (i.e. goodness) of a particular material is determined by the shape into which it is cast.

therefore its goodness is participated, inasmuch as matter participates the form.

By "participated", I think he means, "determined".   Therefore, the substance determines the use.  Again, let's look at an example.  We wouldn't use water as a cup.  Or wood to contain fire.  We could use clay for both of those purposes.

Now the first good and the best--viz. God--is not a participated good, because the essential good is prior to the participated good.

We can't really speak of God as having a use nor as being good for this or that.  God is all good.  All goodness comes from God.  No one determines to what use God is to be put.  God determines to what use everything else is to be put.  God is the First Cause.

Hence it is impossible that God should be composed of matter and form.

Since God is the First Cause, then it is impossible that He should be limited by a body.

Thirdly, because every agent acts by its form; hence the manner in which it has its form is the manner in which it is an agent. Therefore whatever is primarily and essentially an agent must be primarily and essentially form. Now God is the first agent, since He is the first efficient cause. He is therefore of His essence a form; and not composed of matter and form.

God is all good and therefore can not be limited to a particular substance which would limit Him to a particular use.  God is infinite and thus formless.

Reply to Objection 1. A soul is attributed to God because His acts resemble the acts of a soul; for, that we will anything, is due to our soul. Hence what is pleasing to His will is said to be pleasing to His soul.

God is said to be a soul because, a soul's actions resemble God's actions.  Therefore, that which is pleasing to His will is said to be pleasing to His soul.  I believe this is called a simile.  The actions resemble each other, but they are not the same.  Therefore, God does not have a soul.

Reply to Objection 2. Anger and the like are attributed to God on account of a similitude of effect. Thus, because to punish is properly the act of an angry man, God's punishment is metaphorically spoken of as His anger.

In the same way, anger and passion are attributed to God in the same way.  Metaphorically.

Reply to Objection 3. Forms which can be received in matter are individualized by matter,

Shapes and molds which can be given to substances, are made unique by those substances.

which cannot be in another as in a subject since it is the first underlying subject;

 And once a substance is thus shaped it is made unique and can not be duplicated.
*As in, no two snowflakes are alike.  

although form of itself, unless something else prevents it, can be received by many.

We all recognize the shape and form of a snowflake.  Therefore, many individuals can receive the same form.  But each one is unique in some way.

But that form which cannot be received in matter, but is self-subsisting, is individualized precisely because it cannot be received in a subject; and such a form is God. Hence it does not follow that matter exists in God.

But there is one Shape which can not be received by any substance because it is already Unique.  For there is only one God.