Having shown that God is not the being of all things, we can likewise show that He is not the form of any thing.
 As we have shown, the divine being cannot belong to any quiddity that is not being itself. [Nam divinum esse non potest esse alicuius quidditatis quae non sit ipsum esse, ut ostensum est.] Now, only God is the divine being itself. It is impossible, therefore, for God to be the form of some other being.
 Furthermore, the form of a body is not the being itself, but a principle of being. But God is being itself. [Forma corporis non est ipsum esse, sed essendi principium. Deus autem est ipsum esse.] He is, therefore, not the form of a body.
Here, again, Thomas reminds us that "what a thing is" does not include or entail "that a thing is." Only that which is being itself enjoys the so to speak alignment of what it is with the fact that it is. Since any thing's form––or, quidditas––is distinct from its specific act of being, and since God is the pure act of being, God's power must be supperadded to a thing's form to account for its particular existence. This, however, means that God is not any thing's form, otherwise any thing's form would include its act of existence, which we have just noted does not occur except in God.
...  Moreover, that which has being through itself is nobler than that which has being in another. But every form of a body has being in another [Omnis autem forma alicuius corporis habet esse in alio]. [That is, a finite essence is made actual by way of existing in an actual substance. The form of "catness" does not enjoy existence outside of substantial cats.] Since, then, God, as the first cause of being, is the noblest being, He cannot be the form of any being.
 The same conclusion can also be reached in the following way from the eternity of motion. If God is the form of some movable body, since He is the first mover, the composite will be self-moving. But something self-moving can be moved and not-moved. Both possibilities are found in it. But such a being does not of itself have an indefectibility of motion. [Sed movens seipsum potest moveri et non moveri. Utrumque igitur in ipso est. Quod autem est huiusmodi, non habet motus indeficientiam ex seipso.] Above the self-moving being, therefore, we must posit another first mover, which gives to the self-moving being the endlessness of its motion. Thus, God, Who is the first mover, is not the form of a self-moving body.
If I understand him, Thomas is saying this: a moving body can move and can cease to move. If God were the form of eternally mobile things ("bodies"), however, they would have a completely immobile form as their principle of being, which means they would lack the principle of self-motion. Hence, God, as the immobile mover, is not the form of mobile beings, even if they are eternal.
 This argumentation is suitable for those who posit the eternity of motion. Those who do not posit it can reach the same conclusion from the regularity of the motion of the heavens. For just as a self-mover can be at rest and in motion, so it can be moved more swiftly and less so. The necessity in the uniformity of the motion of the heavens, therefore, depends on some higher and absolutely immobile principle, which is not a part of a self-moving body as the form of that body.
This argument seems so closely tied to Aristotelian physics that I wonder if it is worth saving.
…  Thus, then, is removed the error of the Gentiles, who said that God is the soul of the heavens, or even the soul of the whole world. … On the basis of this error the Gentiles thought it to follow that, not unfittingly, divine worship should be shown to the world and its parts. … [Sic igitur gentilium error evacuatur, qui dicebant Deum esse animam caeli vel etiam animam totius mundi…. quo supposito, sequi opinabantur quod mundo et partibus eius non indebite divinus cultus exhibeatur.]
Interestingly enough, the abridged version of SCG by Fr Rickaby does not include this chapter, so, poor reader, I have had to exegete it by my own lowly wits! Nonetheless, I think this chapter is just a subset, or a smallish consequence, of the previous chapter, which is probably why Fr Rickaby excluded it. It is profitable to consider a note by Rickaby from the previous chapter, since it broadly applies to this chapter:
If all things agreed in being -- and that the divine being -- all things would agree also in nature, since the being of God is simply identical with His nature. Agreeing at once in being and in nature, they would agree all over, all would be absolutely one, and one great and sole Reality would pervade and constitute the universe. To erect such a 'Reality,' or 'Idea,' or 'Absolute,' and then to proclaim it God, is pantheism. St Thomas argues that this all-pervading entity is not the universe, still less is it God: it has no concrete existence whatever: it is the shallowest, poorest and barest of the mind's creations, extending to and denoting everything, and therefore meaning and comprehending next to nothing. In its fourth canon [24 April 1870; DS 1804], De Deo Creatore, the Vatican Council anathematises any who say that "God is a universal or indefinite being, which by self-determination constitutes the universe."