This reason is a response to the unbeliever who challenges the believer for a reason to believe. For it is absurd for an unbeliever to seek to understand the articles of faith without first having the preparation of faith. As for the believer, the use of reason in instruction ought to depend on the ability of the believer, who should never let reason undermine the faith he already has. The end result is "a great understanding of incorporeal and unchanging things" (as also described by Plato) but an encounter with God ("face to face"). Some people who have knowledge of the incorporeal and unchanging cannot make the last step, because they lack faith.
There is reasoning which denies the central orthodox beliefs of the Christian faith (which was identical to the Catholic faith at that time). An example of such unorthodoxy is the claim that the son of God is distinct from the father, e.g. as being literally created by God. It is false reasoning which leads to these errors, and reason is not to be despised because of it. Just as we do not condemn speech because some speech is false, we should not condemn reason simply because it is sometimes wrongly used.
So there are various possible ways of relating faith and reason. True reason is better than false reason, but it by itself cannot reach the truth. One needs faith in what is not understood in order that it may yet be understood. Faith can "see with its own eyes" what is true, though it cannot understand the truth of what it sees.
Faith is not so opposed to reason as it seems. We all believe in things past, and we think such belief is rational. But we cannot view them anymore; they are gone forever. Neither can the future be experienced, so in a sense it has the same status as the past. We hope for things to come, such as our rebirth after death. Other things are never experienced through sense, such as the Platonic forms of justice and wisdom, yet they are "seen" by the understanding "in a special and appropriate manner." As Plato stated, they are understood better than sensible things. Jesus was special in that he is unchangeable and yet was visible.
For those things which are unseen, we can imagine what they would be like, as a city we have never seen. With respect to eternal things like justice, there is no image to be formed. They are known "by a simple intellectual attention of the mind or reason, without any forms or physical bulk, without any features or appearance of parts, without any locality, whether limited or of unbounded space." We understand them through a "light" which "shines invisibly and indescribably, yet intelligibly." We know of the existence of this "light" as certainly as we know of that which illuminates.
So there are three kinds of object of knowledge: the sensible, the imagined, and the intelligible. The question arises as to the status of the Trinity. It could only be intelligible, not sensible or imaginable.
But the Trinity may be higher even than the intelligible forms. If so, then it is incomparably higher than anything sensible.