Paraphrasing the Summa: First part, Question 1, Article 9

Question 1: The nature and extent of sacred doctrine.

Is Christian doctrine revealed by God?  Does it teach us about the supernatural as well as the natural?

Article 9. Whether Holy Scripture should use metaphors?

Should Christian doctrine ever use figures of speech?

Objection 1. It seems that Holy Scripture should not use metaphors. For that which is proper to the lowest science seems not to befit this science, which holds the highest place of all. But to proceed by the aid of various similitudes and figures is proper to poetry, the least of all the sciences. Therefore it is not fitting that this science should make use of such similitudes.

It is not in accordance with the nobility of something revealed by God that it should be necessary to express its ideas indirectly.  Rather, Divine Revelation should be explicit and clear.  Otherwise, it is not an expression of God but an expression of an imperfect creature.

Objection 2. Further, this doctrine seems to be intended to make truth clear. Hence a reward is held out to those who manifest it: "They that explain me shall have life everlasting" (Sirach 24:31). But by such similitudes truth is obscured. Therefore, to put forward divine truths by likening them to corporeal things does not befit this science.

 Christian doctrine should be clearly expressed in order to properly represent truth.  But figures of speech tend to make truth hard to understand.  Therefore, to compare heavenly things to common things of this world does not help in making Christian doctrine understandable.

Objection 3. Further, the higher creatures are, the nearer they approach to the divine likeness. If therefore any creature be taken to represent God, this representation ought chiefly to be taken from the higher creatures, and not from the lower; yet this is often found in Scriptures.

In addition, God should never be represented by the lower forms of life.  Yet, we often find this in Sacred Scripture.

On the contrary, It is written (Hosea 12:10): "I have multiplied visions, and I have used similitudes by the ministry of the prophets." But to put forward anything by means of similitudes is to use metaphors. Therefore this sacred science may use metaphors.

It is precisely the opposite, it is in figures and visions that God normally communicates with humans.  Therefore, Christian doctrine may contain metaphors.

I answer that, It is befitting Holy Writ to put forward divine and spiritual truths by means of comparisons with material things. For God provides for everything according to the capacity of its nature.

Not only is it permissible, but it is necessary that Scripture communicate divine truths by comparing to things of this world.  Otherwise, mankind would not be able to comprehend that which was being communicated.

Now it is natural to man to attain to intellectual truths through sensible objects, because all our knowledge originates from sense.

Man's understanding must begin with that which he can see and feel.

Hence in Holy Writ, spiritual truths are fittingly taught under the likeness of material things.

Therefore, Scripture begins by comparing invisible truths to things which can be seen and felt.

This is what Dionysius says (Coel. Hier. i): "We cannot be enlightened by the divine rays except they be hidden within the covering of many sacred veils."

Therefore, Dionysius explains that God's grace is given us by means of material things (i.e. the Sacraments).

It is also befitting Holy Writ, which is proposed to all without distinction of persons — "To the wise and to the unwise I am a debtor" (Romans 1:14) — that spiritual truths be expounded by means of figures taken from corporeal things, in order that thereby even the simple who are unable by themselves to grasp intellectual things may be able to understand it.

And Scripture communicates spiritual truths by means of comparisons to common things in order that  these ideas be comprehended by the simplest minds.

Reply to Objection 1. Poetry makes use of metaphors to produce a representation, for it is natural to man to be pleased with representations. But sacred doctrine makes use of metaphors as both necessary and useful.

 To the first objection, St. Thomas says, poetry is a discipline but not a science.  It is an art which does not necessarily seek to convey truth, but primarily seeks to entertain.  Therefore, it is improper to compare poetry to Christian doctrine.

However, Christian doctrine uses figures of speech in order to convey truth in a manner that is understood by the simplest of minds.

Reply to Objection 2. The ray of divine revelation is not extinguished by the sensible imagery wherewith it is veiled, as Dionysius says (Coel. Hier. i); and its truth so far remains that it does not allow the minds of those to whom the revelation has been made, to rest in the metaphors, but raises them to the knowledge of truths;

The message of God is not destroyed by the common language in which it is conveyed.  The truth is conveyed by common means in order to move the human mind from earthy knowledge to understanding the will of God.

and through those to whom the revelation has been made others also may receive instruction in these matters.

God's message was given to men in order that these men would then convey the message to others.

Hence those things that are taught metaphorically in one part of Scripture, in other parts are taught more openly. 

But God's message is not limited to figures.  In some portions, Scripture makes a point figuratively, but in another place, expressly.

The very hiding of truth in figures is useful for the exercise of thoughtful minds and as a defense against the ridicule of the impious, according to the words "Give not that which is holy to dogs" (Matthew 7:6).

 In addition, God's truths are not for all.  But only for those who seek God and want to be united to Him.  As the Scripture says, "Give not that which is holy to dogs" (Matthew 7:6). *

Reply to Objection 3. 

Objection 3 says that divine revelation should never be explained in figures of speech.

As Dionysius says, (Coel. Hier. i) it is more fitting that divine truths should be expounded under the figure of less noble than of nobler bodies, and this for three reasons.

St. Thomas gives three reasons why this is not so.  These three were first expressed by St. Dionysius.

Firstly, because thereby men's minds are the better preserved from error. For then it is clear that these things are not literal descriptions of divine truths, which might have been open to doubt had they been expressed under the figure of nobler bodies, especially for those who could think of nothing nobler than bodies.

First, because it is easier for men to comprehend and therefore, less likely for men to misunderstand.

Secondly, because this is more befitting the knowledge of God that we have in this life. For what He is not is clearer to us than what He is. Therefore similitudes drawn from things farthest away from God form within us a truer estimate that God is above whatsoever we may say or think of Him.

2nd, because the essence of God is knowledge which our mind can not comprehend.  Therefore, by giving examples of that which God is not, we come to a better understanding of God's essence.

Thirdly, because thereby divine truths are the better hidden from the unworthy.

And finally, because God's truths are not for everyone but only to those who have faith in God and want to please Him.