Ch. 16 "God Not a Body" from "The Compendium of Theology" by St. Thomas Aquinas

Caput 16 Quod impossibile est Deum esse corpus

CHAPTER 16 GOD NOT A BODY

Patet autem ulterius quod impossibile est ipsum Deum esse corpus. Nam in omni corpore compositio aliqua invenitur: omne enim corpus est partes habens. Id igitur quod est omnino simplex, corpus esse non potest.

It is evident, further, that God Himself cannot be a body. For in every body some composition is found, since a body has parts. Hence that which is absolutely simple cannot be a body.

Item. Nullum corpus invenitur movere nisi per hoc quod ipsum movetur, ut per omnia inducenti apparet. Si ergo primum movens est omnino immobile, impossibile est ipsum esse corpus.

Moreover, we find that a body does not move anything else unless it is first moved itself, as will appear clearly to anyone who examines the matter fully. So if the first mover is absolutely immovable, that being cannot be a body.

Chapter sixteen is very brief, but Aquinas makes the simple point that God cannot have a body. He says this because bodies are always composed of parts, by definition of having quantity, and thus is antithetical of the metaphysically simple, which he previously showed God to be. Likewise, anything with composition of parts is also subject to an act - potency relationship between the parts, and thus to movement, while, again, God has shown to be immobile.