On the contrary, It is written in the Gospel of St. John (John 4:24): "God is a spirit."
I answer that, It is absolutely true that God is not a body; and this can be shown in three ways.
First, because no body is in motion unless it be put in motion, as is evident from induction. Now it has been already proved (2, 3), that God is the First Mover, and is Himself unmoved. Therefore it is clear that God is not a body.
Secondly, because the first being must of necessity be in act, and in no way in potentiality. For although in any single thing that passes from potentiality to actuality, the potentiality is prior in time to the actuality; nevertheless, absolutely speaking, actuality is prior to potentiality; for whatever is in potentiality can be reduced into actuality only by some being in actuality. Now it has been already proved that God is the First Being. It is therefore impossible that in God there should be any potentiality. But every body is in potentiality because the continuous, as such, is divisible to infinity; it is therefore impossible that God should be a body.
Thirdly, because God is the most noble of beings. Now it is impossible for a body to be the most noble of beings; for a body must be either animate or inanimate; and an animate body is manifestly nobler than any inanimate body. But an animate body is not animate precisely as body; otherwise all bodies would be animate. Therefore its animation depends upon some other thing, as our body depends for its animation on the soul. Hence that by which a body becomes animated must be nobler than the body. Therefore it is impossible that God should be a body.
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.
Thomas Aquinas, Summa Theologica