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.
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.
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.
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.
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.
Secondly, because everything composed of matter and form owes its perfection and goodness to its form; therefore its goodness is participated, inasmuch as matter participates the form. Now the first good and the best--viz. God--is not a participated good, because the essential good is prior to the participated good. Hence it is impossible that God should be composed of matter and form.
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.
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.
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.
Reply to Objection 3. Forms which can be received in matter are individualized by matter, which cannot be in another as in a subject since it is the first underlying subject; although form of itself, unless something else prevents it, can be received by many. 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.
Source: Summa Theologica (New Advent)