This is from I Sent pr. q. 1 a. 1. Standard caveats about roughness of translation apply. The Latin is here. The article is a bit more messy than what you get in the Summa, but there are some interesting things here.
To the fifth we proceed in this way.
It seems that the mode of proceeding is not artful. For the most noble science ought to have the most noble mode. But by how much more a mode is artful, by so much it is more noble. Therefore, as this science is the most noble, its mode ought to be the most artful.
(2) Further, the mode of a science ought to be proportionate to the science itself. But this is maximally one, as was proven. Therefore also its mode ought to be maximally unitary. But the contrary seems to be the case, for sometimes it proceeds by warning, sometimes by legislating, and sometimes by some other mode.
(3) Further, sciences maximally differing ought not to have one mode. But poetics, which contains the minimum of truth, maximally differs from this science, which is most true. Therefore, as the former proceeds by metaphorical locutions, the latter's mode of science ought not to be like it (non debet esse talis).
(4) Further, Ambrose: "Away with arguments where faith is sought." But in the sacred science faith is most sought (maxime quaeritur). Therefore its mode ought in no way to be argumentative.
Against this. 1 Pet. 3:15: Be ready always with an answer to everyone who asks a reason for the hope that is in you. But this without argument ceases to avail (fieri non valet). Therefore it ought sometimes to use arguments.
The same is found in that which is said in Tit. 1:9: That he may be able both to exhort in sound doctrine and to argue against those who contradict it.
I respond that it must be said, that one should inquire into the mode of any science according to the conditions of its matter, as says Boethius, and also the Philosopher. The principles of this science are accepted by revelation; and thus the mode of accepting these principles ought to be revelatory on the part of the one infusing, as in the revelations of the Prophets, and prayerful on the part of those receiving it, as is clear in the Psalms. But because, apart from infused light, it is fitting that the habit of faith be disinguished into determinate beliefs by the doctrine of the preacher, as is said in Rom. 10:14 (How will they believe who have not heard?) just as the understanding of principles instilled naturally is determined by accepted sensibles, so the truth of the preacher is confirmed by miracles, as says Mark 16:20 (But they went forth and preached everywhere, while the Lord worked with them and confirmed the preaching by the signs that followed.), it is fitting then for the mode of this science to be narrative of signs, which are done for the confirmation of the faith; and, because these principles are not proportioned to human reason according to its state in life, which is accustomed to accept things from sensibles, it is thus fitting that the cognition of these things be drawn out by way of sensible similitude; wherefore it is fitting that the mode of this science be metaphorical, or symbolic or parabolic.
In sacred Scripture proceeding from these principles leads to three things: first to the destruction of errors, which cannot be done without arguments, and thus it is fitting that the mode of this science be sometimes argumentative, both through authorities and also through reasons or natural similitudes.
It also proceeds to the instruction in morals: wherefore its mode ought to be preceptive, as in the Law; admonitory and promissory, as in the Prophets; and narrative of examples, as in the Historical Books.
It proceeds in a third way to the contemplation of truth in the questions of sacred Scripture; and to this end it is fitting that its mode also be argumentative, which is especially contained in the original saints and in this book [i.e., Lombard's Sentences], which as it were conjoins them.
And according to this there likewise can be accepted a fourfold mode of expounding sacred Scripture: because according as the truth of faith is accepted, it is the historical sense; according as it is that which proceeds to instruction in morals, it is the moral sense; according as it proceeds to the contemplation of the truth of things on the way, it is the allegorical sense; and according as it proceeds to the contemplation of those things to which we are destined (quae sunt patriae), it is the anagogical sense. But to the the destruction of error one does not proceed save through the literal sense, in that the other senses are through accepted similitudes, and from similitudes of speech argumentation cannot be drawn; wherefore Dionysius says (in the epistle to Titus, in the beginning) that symbolic theology is not argumentative.
To the first it must therefore be said that a mode is said to be artful which agrees with the matter; wherefore the mode that is artful in geometry is not artful in ethics: and according to this the mode of this science is maximally artful, because it is maximally congruent to its matter.
To the second it must be said, that although this science is one, nonetheless it is of many things and avails for many things (ad multa valet), according to which it is fitting for its modes to be multiplied, as is already clear.
To the third it must be said, that the poetic science is of things that according to a defect of truth are not able to be grasped rationally; wherefore it is fitting that reason be as it were seduced by similitudes: but theology is of things that are above reason; and thus the symbolic mode is common to them both, for neither is proportionate to reason.
To the fourth it must be said, that arguments for proving the articles of faith are destroyed; but for the defense of the faith and the discovery of truth in questions from the principles of faith, it is fitting for arguments to be used: and so the apostle does, 1 Corinth. 15:16: if Christ is resurrected, so also the dead rise.
Update (28 Feb): I've done a bit of tweaking here and there.