something which is predicated of the whole may be predicated of a part
(as a part of the air is air, and a part of water, water),
nevertheless certain things are predicable of the whole which cannot
be predicated of any of the parts; for instance, if the whole volume
of water is two cubits, no part of it can be two cubits. Thus in every
composite there is something which is not it itself. But, even if this
could be said of whatever has a form, viz. that it has something which
is not it itself, as in a white object there is something which does
not belong to the essence of white; nevertheless in the form itself,
there is nothing besides itself. And so, since God is absolute form,
or rather absolute being, He can be in no way composite. Hilary
implies this argument, when he says (De Trin. vii): "God, Who is
strength, is not made up of things that are weak; nor is He Who is
light, composed of things that are dim."

Reply Obj. 1: Whatever is from God imitates Him, as caused
things imitate the first cause. But it is of the essence of a thing to
be in some sort composite; because at least its existence differs from
its essence, as will be shown hereafter, (Q. 4, A. 3).

Reply Obj. 2: With us composite things are better than simple
things, because the perfections of created goodness cannot be found in
one simple thing, but in many things. But the perfection of divine
goodness is found in one simple thing (QQ. 4, A. 1, and 6, A. 2).
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EIGHTH ARTICLE [I, Q. 3, Art. 8]

Whether God Enters into the Composition of Other Things?