Paraphrasing the Summa: First part, Question 3, article 7

Article 7. Whether God is altogether simple?

 *This is not asking whether God is gullible or stupid.  It is asking whether God is complex or composed of various parts, like an organism.  So, we can phrase it differently.

Is God an organism?  Is God made of component parts?

Objection 1. It seems that God is not altogether simple. For whatever is from God must imitate Him. Thus from the first being are all beings; and from the first good is all good. But in the things which God has made, nothing is altogether simple. Therefore neither is God altogether simple.

The first objection says that since all things which God has made are evidence of God's existence.  They point to God and are somehow, like God.  And God has not made anything which does not have component parts.  Therefore, God must also be made of component parts.  So, God must be complex.

Objection 2. Further, whatever is best must be attributed to God. But with us that which is composite is better than that which is simple; thus, chemical compounds are better than simple elements, and animals than the parts that compose them. Therefore it cannot be said that God is altogether simple.

Simple organisms are considered the least of the creatures of earth.  Complex organisms, like humans, are considered higher beings in the evolutionary chain.  Therefore, since the best of all things is attributed to God, God must be the most complex Being of all.

On the contrary, Augustine says (De Trin. iv, 6,7): "God is truly and absolutely simple."

St. Thomas refers to St. Augustine who, in his book "on the Trinity", said, "God is truly and absolutely simple".

I answer that, The absolute simplicity of God may be shown in many ways.

He continues to say that God is lacking in any complexity whatsoever and this can be proven...

First, from the previous articles of this question. For there is neither composition of quantitative parts in God, since He is not a body; nor composition of matter and form; nor does His nature differ from His "suppositum"; nor His essence from His existence; neither is there in Him composition of genus and difference, nor of subject and accident. Therefore, it is clear that God is nowise composite, but is altogether simple.

In  Question 3, Article 2, we discussed that God has no substance.  We also discussed in the previous article (Question 3, Article 6) that God has no unnecessary properties (accidents).  Therefore, we know that God is not complex at all, but absolutely .

Secondly, because every composite is posterior to its component parts, and is dependent on them; but God is the first being, as shown above (Question 2, Article 3).

Second, all complex things are put together.  They depend upon their parts.  Therefore, their parts must precede them. But God is the First Cause and nothing precedes Him.

Thirdly, because every composite has a cause, for things in themselves different cannot unite unless something causes them to unite. But God is uncaused, as shown above (Question 2, Article 3), since He is the first efficient cause.

All complex things must be put together by an external force, they can't build themselves.  But God is the First Cause and Himself was not made or created by any other.

Fourthly, because in every composite there must be potentiality and actuality; but this does not apply to God; for either one of the parts actuates another, or at least all the parts are potential to the whole.

In complex creatures, one part moves another.  But God is not made of any parts and has no substance.  God is Spirit and Life.  God actuates everything else.

Fifthly, because nothing composite can be predicated of any single one of its parts.  And this is evident in a whole made up of dissimilar parts; for no part of a man is a man, nor any of the parts of the foot, a foot. 

The complex whole can not be predicted by one of its pieces or components.
* An example of this, might be a trunk.  Elephants have trunks.  But so do some mice, seals and other creatures, also have trunks.  So, we can't predict that an animal is an elephant simply because it has a trunk.

But in wholes made up of similar parts, although 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),

But all parts of air are air and all parts of water are 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.

But if you take a cup of water from  a larger quantity, you can't tell whether the amount of the larger quantity.

Thus in every composite there is something which is not it itself.

So, you can't predict the whole from the part.

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.

*What does he mean by, "even if this could be said of whatever has a form...?  That you can't predict the whole from the part?  God is a simple form, as St. Thomas said previously (Question 3, Article 6).

I think what he is doing is separating the substance from the form.  Therefore, he uses the example of a white object.  Now, in a previous article, he spoke of the form as the "goodness" of the object.  Or, as I paraphrased it, the "use" of the object (Question 3, Article 2, I answer that).

Therefore, he seems to be saying that, although the substance is constituted of parts, yet the whole was made for one purpose and all its components serve that one purpose.

And so, since God is absolute form, or rather absolute being, He can be in no way composite.

But God is a simple form.   He is not composite in anyway, so this is even more true of God.  If all parts of a composite being have only one form, then God, who is not composite and has no material, must only be one Form.

 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."

*I understand this better in the way St. Therese of Lisieux (Letters) said it.  She said that "God's justice is pure love and God's love is pure justice."  So, we can go on to understand that God's power is pure love and God's love is pure power.  Etc. etc.   God is not one part power, another part justice and another part love.  God doesn't love at one time and stop loving at another.   Even when we are being punished, God is loving us.  

Reply to Objection 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, (4, 3).

In Question 4, Article 3, St. Thomas will show that created things must, by nature, be composite.

Reply to Objection 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 (4, 1 and 6, 2).

Objection 2 said that composite things are more perfect than simple things.  St. Thomas counters that created things are made more perfect by additional improvements added to them.  But God's perfection is absolute.  Again, he will prove this in upcoming lessons.