Paraphrasing the Summa: First Part, Question 1: Article 3

Question 1:  The nature and extent of Sacred Doctrine

Is Christian doctrine from God?  Does it teach us about heavenly things?

Article 3. Whether sacred doctrine

 is one science?

Is Christian doctrine one body of knowledge or a mixture of subjects?

Objection 1. It seems that sacred doctrine is not one science; for according to the Philosopher (i.e. Aristotle) "that science is one which treats only of one class of subjects." But the creator and the creature, both of whom are treated of in sacred doctrine, cannot be grouped together under one class of subjects. Therefore sacred doctrine is not one science.

The first objection seeks to disqualify Christian doctrine as a science on the grounds that Christian doctrine is not strictly about one subject.  But, about two.  It is about God and about man.

Objection 2. Further, in sacred doctrine we treat of angels, corporeal creatures and human morality. But these belong to separate philosophical sciences. Therefore sacred doctrine cannot be one science.

The second objection adds to the first that there are even more subjects treated in Christian doctrine, such as angels and animals as well as ethics and justice.  Therefore, the objection concludes that Christian doctrine is a mixture of disciplines and not specifically one.

On the contrary, Holy Scripture speaks of it as one science: "Wisdom gave him the knowledge [scientiam] of holy things" (Wisdom 10:10).

The Angelic Doctor objects to the objection by a reference to Scripture.  Wherein,  it says that Christian doctrine gives us the knowledge (i.e. science) of God's will.  Therefore, since Scripture regards Christian doctrine as one body of knowledge, it is one science.

I understand "holy things" to mean, "those things which God wills us to do and to know".  

I answer that, Sacred doctrine is one science.

St. Thomas emphatically states that Christian doctrine is one body of knowledge.

The unity of a faculty or habit is to be gauged by its object, not indeed, in its material aspect, but as regards the precise formality under which it is an object.

Wow!  That's complicated.  Let's break it down.

 A faculty, like sight.  Sight is the habitual faculty of the eye.  The eye constantly sees things without having to remember to do so.

The unity of a faculty?  In this context, we are not considering the faculty of sight.  We are considering the faculty of understanding.  I don't know what he means by "unity".  Perhaps, cohesiveness.  But let's move on.

Gauged - measured, evaluated.
Its object -  What is it that we are trying to understand?  Christian Doctrine.
Material aspect -  form
Precise formality? - In order to understand this, we need to answer the question, "What form does Christian doctrine take?"  It is passed down to us in teaching, by word and by epistle.

What quality do both the word and epistle share?  They are divinely inspired.

So, let me try to paraphrase that complicated sentence.

We are better able to understand Christian Doctrine, not by studying the immediate and human source of the teaching (i.e material aspect) , but by recognizing that it is revealed to us by God (the precise formality of being divinely revealed).

 For example, man, ass, stone agree in the one precise formality of being colored; and color is the formal object of sight.  Therefore, because Sacred Scripture considers things precisely under the formality of being divinely revealed,

In this sentence, St. Thomas is equating Sacred Scripture with Christian doctrine.  And he says that Sacred Scripture evaluates holy things precisely as sight evaluates colors.  Which corresponds to what we said above.

whatever has been divinely revealed possesses the one precise formality of the object of this science; and therefore is included under sacred doctrine as under one science.

Therefore, the evaluation of holy things is the object of this body of knowledge. Therefore, the evaluation of holy things is the science of Christian doctrine.
And therefore, Christian doctrine is one science dedicated to the evaluation of that which is divinely revealed.

Reply to Objection 1. Sacred doctrine does not treat of God and creatures equally, but of God primarily, and of creatures only so far as they are referable to God as their beginning or end. Hence the unity of this science is not impaired.

The first objection to treating Christian doctrine as a science is that science is about one thing.  Whereas Christian doctrine is about several.

St. Thomas denies this and explains that Christian doctrine is mainly about God. And makes reference to others only with respect to God as their Maker or Savior.  Therefore, Christian doctrine is one science.

Reply to Objection 2. 

The second objection says that Christian doctrine treats of many things. Therefore it is not one science.

Nothing prevents inferior faculties or habits from being differentiated by something which falls under a higher faculty or habit as well;

St. Thomas replies that there is no reason why a science should not refer to secondary and inferior attributes and abilities.

because the higher faculty or habit regards the object in its more universal formality, 

The more important abilities or behaviors evaluate data in a general sense.

as the object of the "common sense" is whatever affects the senses, including, therefore, whatever is visible or audible.  Hence the "common sense", although one faculty, extends to all the objects of the five senses.

Normal intelligence (common sense) interprets all experiential data collected by our five senses.  In the same way, Christian doctrine interprets all Divinely revealed data, whether the data be with regards to the natural or supernatural order.

Similarly, objects which are the subject-matter of different philosophical sciences can yet be treated of by this one single sacred science under one aspect precisely so far as they can be included in revelation. So that in this way, sacred doctrine bears, as it were, the stamp of the divine science which is one and simple, yet extends to everything.

In addition, that which is revealed by God can not be limited to supernatural knowledge alone.  Because God has revealed to us much knowledge about the natural world.  Therefore, Christian doctrine is one body of knowledge even though it regards both natural and supernatural elements.