Paraphrasing the Summa: First part, Question 3, article 8

Article 8. Whether God enters into the composition of other things?

Are creatures made from God's Being?

*This is a very powerful question which I submit to the authority of the Catholic Church for whatever the answer might be.  I, don't understand the question or St. Thomas' answer.

Objection 1. It seems that God enters into the composition of other things, for Dionysius says (Coel. Hier. iv): "The being of all things is that which is above being--the Godhead." But the being of all things enters into the composition of everything. Therefore God enters into the composition of other things.

God is said to be all-being.  In Him we live, move and are.  Therefore, creatures are made from God's Being.

Objection 2. Further, God is a form; for Augustine says (De Verb. Dom., [Serm. xxxviii) that, "the word of God, which is God, is an uncreated form." But a form is part of a compound. Therefore God is part of some compound.

In addition, God is a form.  But all forms are attached to substance.  Therefore, God is a compound.
*This one, in my opinion, fits better in the last set of objections (Question 3, Article 7).

Objection 3. Further, whatever things exist, in no way differing from each other, are the same. But God and primary matter exist, and in no way differ from each other. Therefore they are absolutely the same. But primary matter enters into the composition things. Therefore also does God. Proof of the minor--whatever things differ, they differ by some differences, and therefore must be composite. But God and primary matter are altogether simple. Therefore they nowise differ from each other.

*Primary matter is a philosophical concept which includes all material things.  If we ask, what are molecules made of, the answer would be, atoms.  If we ask, what are atoms made of, the answer would be electrons, protons and neutrons.  But what are those made of?  Quarks?  And what are those made of?  Primary matter.  

This objection equates Primary matter with God.  And says that everything is made from God's substance.

On the contrary, Dionysius says (Div. Nom. ii): "There can be no touching Him," i.e. God, "nor any other union with Him by mingling part with part."

St. Thomas quotes St. Dionysius saying that there is no way that a creature can even touch God, much less, mingle his substance with the Divine nature.

Further, the first cause rules all things without commingling with them, as the Philosopher says (De Causis).

And quotes Aristotle saying the same thing.

I answer that, On this point there have been three errors. Some have affirmed that God is the world-soul, as is clear from Augustine (De Civ. Dei vii, 6). This is practically the same as the opinion of those who assert that God is the soul of the highest heaven. Again, others have said that God is the formal principle of all things; and this was the theory of the Almaricians. The third error is that of David of Dinant, who most absurdly taught that God was primary matter. Now all these contain manifest untruth; since it is not possible for God to enter into the composition of anything, either as a formal or a material principle.

Then he makes reference to three errors explained by St. Augustine, wherein, some philosophers have described God as the soul of the universe.  But a soul is a combination of body and spirit.  And God has no substance, thus no body.  And therefore, God is not the soul of the universe.

First, because God is the first efficient cause. Now the efficient cause is not identical numerically with the form of the thing caused, but only specifically: for man begets man. 

First, God is the Creator.   The Creator should not be confused with the creature as though they were one entity.  As a human father begets a human son, they have the same nature.  But, individually, they are separate and should not be confused, they are different individuals.

But primary matter can be neither numerically nor specifically identical with an efficient cause; for the former is merely potential, while the latter is actual.

But primary matter is not the Creator.  It is a creature which was made by the Creator.  It is not the Maker or First Actor, but it is acted upon.

Secondly, because, since God is the first efficient cause, to act belongs to Him primarily and essentially. But that which enters into composition with anything does not act primarily and essentially, but rather the composite so acts; for the hand does not act, but the man by his hand; and, fire warms by its heat. Hence God cannot be part of a compound.

Second, God is the First Cause and by nature, active.  Creatures are compound of matter and spirit.  And they act, basically, because the spirit moves the matter.  As the spiritual soul moves the body in a human being.

Thirdly, because no part of a compound can be absolutely primal among beings--not even matter, nor form, though they are the primal parts of every compound.

Both the substance and the shape are the most important parts of any complex being.

For matter is merely potential; and potentiality is absolutely posterior to actuality, as is clear from the foregoing (3, 1):

Because substance is always at rest and therefore always secondary to that which activates it.

while a form which is part of a compound is a participated form;

While the shape of a substance serves only the purpose for which it is intended.

and as that which participates is posterior to that which is essential,

The only essential or necessary Being is God.  Therefore, he is making a reference to creatures, participated beings, being secondary to the only essential Being, God.

so likewise is that which is participated; as fire in ignited objects is posterior to fire that is essentially such. 

The only fire which we know that is eternal is God, the consuming fire (Heb 12:29).

Now it has been proved that God is absolutely primal being (2, 3).

Therefore, all creatures are secondary to God.

Reply to Objection 1. The Godhead is called the being of all things, as their efficient and exemplar cause, but not as being their essence.

St. Thomas says that although God is considered all being, He is such only as their Creator, not as part of their nature or substance.

Reply to Objection 2. The Word is an exemplar form; but not a form that is part of a compound.

All things are made in the image of God.  But not according to substance or matter.

Reply to Objection 3. Simple things do not differ by added differences--for this is the property of compounds. Thus man and horse differ by their differences, rational and irrational; which differences, however, do not differ from each other by other differences. Hence, to be quite accurate, it is better to say that they are, not different, but diverse. Hence, according to the Philosopher (Metaph. x), "things which are diverse are absolutely distinct, but things which are different differ by something." Therefore, strictly speaking, primary matter and God do not differ, but are by their very being, diverse. Hence it does not follow they are the same.

*I don't really get the distinction he is making between diverse and different.  But I do get the gist of the answer.

God is absolutely different from primal matter.  Primal matter is a physical substance and God is not made of any physical substance.