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

Question 1:  The nature and extent of Sacred Doctrine

As he explained in the Introduction, St. Thomas limited his Summa to understanding the Christian religion.  Therefore, Question 1, asks the question, "what is the nature and extent of the Christian Teaching?"

Or, to put it another way, "Is Christian Teaching of natural origins?  Or is it of God?"

And, how far does it extend?  Is it supposed to teach us only about our world?  Or does it teach us about heavenly realities, as well?

Article 1. Whether, besides philosophy,
any further doctrine is required?

Why do we need to study more than philosphy?  Why do we need to study Christian Teaching?

Objection 1. 

St. Thomas lists the arguments against Christian Teaching.  He calls these, "objections".

It seems that, besides philosophical science, we have no need of any further knowledge. For man should not seek to know what is above reason: "Seek not the things that are too high for thee" (Sirach 3:22). But whatever is not above reason is fully treated of in philosophical science. Therefore any other knowledge besides philosophical science is superfluous.

The first argument against teaching Christian doctrine is that it is beyond man's ability to learn.  Even Scripture warns us not to go beyond what we can understand (Sirach 3:22).  

But, human philosophies are well within human understanding.  They do not exceed the human mind's ability to learn.  Therefore, that is all we need.

Objection 2. Further, knowledge can be concerned only with being, for nothing can be known, save what is true; and all that is, is true. But everything that is, is treated of in philosophical science--even God Himself; so that there is a part of philosophy called theology, or the divine science, as Aristotle has proved (Metaph. vi). Therefore, besides philosophical science, there is no need of any further knowledge.

The second argument against teaching Christian doctrine is that Christian doctrine is really just another branch of philosophy.  Therefore, as this argument goes, there is no need for another category of knowledge.  If it is true, Christian doctrine fits quite well under philosophy, because philosophy teaches all which is true.

On the contrary, 

St. Thomas organizes the Summa in this manner.  First he lists the objections to Catholic Doctrine.  Then he shoots down the objections in one general argument.

He always begins his general argument with the words, "On the contrary". 

It is written (2 Timothy 3:16):

That is a reference to St. Paul's 2nd letter to St. Timothy.  It is in the New Testament.  The quote follows.

"All Scripture, inspired of God 

This verse tells us that holy men of the Church, were inspired by the Holy Spirit, to write the New Testament.

is profitable to teach, to reprove, to correct, to instruct in justice."

Therefore, Scripture is useful for the instructor of the faith when he teaches and admonishes those who are studying the faith.

Now Scripture, inspired of God, is no part of philosophical science, which has been built up by human reason.

Remember that objection 1 said that Christian doctrine falls under the human discipline of philosophy.  But St. Thomas says that Scripture was inspired by God.   So, it is not a human discipline.

Therefore it is useful that besides philosophical science, there should be other knowledge, i.e. inspired of God.

And since it is not a human discipline, then it is a totally separate discipline.  A totally different category of knowledge which does not fall under philosophy and therefore needs to be taught separately.

I answer that, 

St. Thomas elaborates further on his general argument against the objections listed above.

It was necessary for man's salvation that there should be a knowledge revealed by God besides philosophical science built up by human reason.

 Human reason does not teach us how to please God.  Therefore, God had to reveal to mankind what His will is in order that man could obey Him and be saved.

Firstly, indeed, because man is directed to God, as to an end that surpasses the grasp of his reason: "The eye hath not seen, O God, besides Thee, what things Thou hast prepared for them that wait for Thee" (Isaiah 64:4).

Man was made for God.  But man is not capable of knowing this without God's revelation.

But the end must first be known by men who are to direct their thoughts and actions to the end.

Unless man knows that he is made for God, he will not direct himself towards God.

Hence it was necessary for the salvation of man that certain truths which exceed human reason should be made known to him by divine revelation.  

Since Christian doctrine is higher than human philosophies, it needs to be taught to man.

Even as regards those truths about God which human reason could have discovered,

Although man's reason could stumble upon some truths about God.  These would not be sufficient to achieve salvation.

it was necessary that man should be taught by a divine revelation; because the truth about God such as reason could discover, would only be known by a few, and that after a long time, and with the admixture of many errors. Whereas man's whole salvation, which is in God, depends upon the knowledge of this truth. Therefore, in order that the salvation of men might be brought about more fitly and more surely, it was necessary that they should be taught divine truths by divine revelation. It was therefore necessary that besides philosophical science built up by reason, there should be a sacred science learned through revelation.

Therefore it is necessary to teach men, Christian doctrine, in order that men may know what they need to do to please God and be saved.

Reply to Objection 1. 

Now, he gets specific.  He does this throughout his book.  First he lists the objections to Christian doctrine.  Then he contradicts those objections in his general argument which begins with the words, "on the contrary".  Then he elaborates on his general argument beginning with the words, "I answer that".   Then he addresses each objection specifically, beginning with the words, "Reply to objection".

Although those things which are beyond man's knowledge may not be sought for by man through his reason,

Men generally don't care about those things which they can't understand.  Therefore, they don't seek to learn about them.

nevertheless, once they are revealed by God, they must be accepted by faith.

However, once God has made them known, then men must believe that they are true and act upon them.

Hence the sacred text continues, "For many things are shown to thee above the understanding of man" (Sirach 3:25). And in this, the sacred science consists.

And since God has made known to us many things which we are not taught in human philosophies.  It is these things which Christian doctrine teaches.  And because of these things that it is necessary to teach Christian doctrine.

Reply to Objection 2. Sciences are differentiated according to the various means through which knowledge is obtained. For the astronomer and the physicist both may prove the same conclusion: that the earth, for instance, is round: the astronomer by means of mathematics (i.e. abstracting from matter), but the physicist by means of matter itself. Hence there is no reason why those things which may be learned from philosophical science, so far as they can be known by natural reason, may not also be taught us by another science so far as they fall within revelation. Hence theology included in sacred doctrine differs in kind from that theology which is part of philosophy.

There is a redundancy in human knowledge.  Some things can be independently discovered in separate disciplines.  Therefore, the astronomer and the physicist both discovered that the earth is round using different means.  Yet, no one argues that one of those branches of knowledge is superfluous.

Therefore, although men can learn certain things about Christian doctrine within the discipline of philosophy.  Men can learn those and more things about Christian doctrine in a separate discipline of Theology.