Summa contra Gentiles1.27 Chapter XXVII. That God is not the form of a body

Accordingly, having shown that God is not the being of all, it can be proved in like manner that God is not the form of any thing.

For the divine being cannot be the being of a quiddity that is not its own being, as shown above. Now that which is the divine being itself is no other than God. Therefore it is impossible for God to be the form of any other thing.

Further. The form of a body is not its very being but the principle of its being. But God is being itself. Therefore God is not the form of a body.

Again. The union of form and matter results in a composite, and this is a whole in respect of form and matter. Now the parts are in potentiality with respect to the whole: but in God there is no potentiality. Therefore it is impossible for God to be the form united to any thing. 1 Ch. xxii. a Ch. xvi.

Again. That which has being per se, is more excellent than what has being in another. Now every form of a body has being in another. Since then God is the most excellent being, as the first cause of being, He cannot be the form of any thing.

Moreover, this can also be proved from the eternity of movement, as follows. If God were the form of a movable thing, since He is the first mover, the composite will be its own mover. But that which moves itself can be moved and not moved. Therefore it is in it to be either. Now a thing of this kind has not of itself indefectibility of movement. Therefore above that which moves itself we must place something else as first mover, which confers on it perpetuity of movement. And thus God Who is the first mover is not the form of a body that moves itself.

This argument avails for those who hold the eternity of movement. Yet if this be not granted the same conclusion may be drawn from the regularity of the heavenly movement. For just as that which moves itself can both be at rest and be moved, so can it be moved with greater or less velocity. Wherefore the necessity of uniformity in the heavenly movement depends on some higher principle that is altogether immovable, and that is not the part, through being the form, of a body which moves itself.

The authority of Scripture is in agreement with this truth. For it is written in the psalm: Thy magnificence is elevated above the heavens; and (Job xi. 8, 9): He is higher than heaven, and what wilt thou do?... the measure of Him is longer than the earth, and deeper than the sea.

Hence we are able to refute the error of the pagans who asserted that God was the soul of the heaven or even the soul of the whole world: which led them to defend the idolatrous doctrine whereby they said that the whole world was God, not in reference to the body but to the soul, even as man is said to be wise in reference not to his body but to his soul: which being supposed they deemed it to follow that divine worship is not unduly shown to the world and its parts. The Commentator also says (Metaph. xi.) that this occasioned the error of the Zabian people, i.e. of idolaters, because, to wit, they asserted that God was the soul of the heaven.