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.
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.
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.
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.
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. Now it is natural to man to attain to intellectual truths through sensible objects, because all our knowledge originates from sense. Hence in Holy Writ, spiritual truths are fittingly taught under the likeness of material things. 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." 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.
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.
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; and through those to whom the revelation has been made others also may receive instruction in these matters. Hence those things that are taught metaphorically in one part of Scripture, in other parts are taught more openly. 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).
Reply to Objection 3. 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. 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. 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. Thirdly, because thereby divine truths are the better hidden from the unworthy.
Source: Summa Theologica (New Advent)