Here are some key points from the first section (Relation of Faith to Reason) of Geisler’s article “Faith and Reason” (Baker Encyclopedia of Christian Apologetics, p. 239; hereafter: BECA):

• The contents of faith “are above reason.” and so must be revealed to humans by God.
• Faith “involves will (freedom) and reason doesn’t coerce the will”.
• Some theological truths “have been proved demonstratively” and can be based on reason, such as the existence of God.

If we take the second point in a straightforward manner, then there appears to be no conflict between faith and reason, at least in terms of the requirement that the assent of faith be a free choice.  If reason doesn’t compel a person to give assent to any theological claim or doctrine (or against any such claim or doctrine), then reason doesn’t preclude a person from freely choosing to give assent to any theological claim or doctrine.

The question I’m not clear how to answer is this:  Can a demonstrative proof compel a person to assent to the conclusion of the proof?  always? sometimes? never?  What is Aquinas’ view on this question?  I’m not sure.

The second section of the article is about Three Uses of Reason:  1. Reason can be used to prove the “preambles of faith”, such as the existence of God.  2. Reason can be used to explain or clarify a theological concept or doctrine.  3. Reason can be used to defend a theological belief by refuting an objection or an argument against that belief. (BECA, p.239).

Although the “preambles of faith” can be “proved demonstratively”(Summa Theologica, 1a.3.2), “such arguments are not available for the second kind of divine truth…” (Gentiles, 1.9, quoted in BECA, p.239).  An example of the second kind of divine truth would be the doctrine of the Trinity.  Aquinas believed that reason alone was insufficient to discover or to prove the doctrine of the Trinity, and that humans can possess this truth only because God has revealed this to us.

The passage quoted from Gentiles 1.9 also includes the following comments about divine revelation:

The sole way to overcome an adversary of divine truth is from the authority of Scripture–an authority divinely confirmed by miracles.  For that which is above the human reason we believe only because God has revealed it.  Nevertheless, there are certainly likely [probable] arguments that should be brought forth in order to make divine truth known.

Apparently when Aquinas speaks of “divine truth” here, he is speaking of the second kind of divine truth (e.g. the doctrine of the Trinity) and not the first kind of divine truth (e.g.  the existence of God), for the first kind of divine truth can be “proved demonstratively” and based on reason alone.

When Aquinas speaks of using “likely [probable] arguments” to support the second kind of divine truth I think he means showing the divine authority of the scriptures (or of Jesus or of the apostles) by making an appeal to the occurence of miracles that allegedly confirm the messages or teachings from those sources.  The authority of the scriptures or revelation is also supported by the belief that God is completely truthful:

…it was necessary for divine truth to be delivered by way of faith, being told to them as it were, by God Himself Who cannot lie…(Summa Theologica, 2a2ae.1, 5.4).

So, Aquinas suggests the basic apologetic argument for the authority of scripture:  Miracles confirm that a book or a messenger are bringing a message that truly comes from God, and God is completely truthful, so we can have complete confidence in the truth and accuracy of books or messengers that have been confirmed by miracles.

Apparently, Aquinas does not see this as a demonstrative proof, but rather as a “likely [probable]” argument. Nevertheless, it is still an argument, a bit of reasoning.  Thus, it seems to me that even the second kind of “divine truth”, such as the doctine of the Trinity, ultimately rests on reason, in that it rests on an argument, a bit of reasoning, about the alleged divine inspiration of the scriptures.  The appeal to confirming miracles requires empirical evidence and the evaluation of that evidence, and the important premise that God is completely truthful, is, presumably, based on a “demonstrative” proof about God being a perfectly good person.