So far we have taken learning as the positive activity of the intellect as it abstracts from material objects and comes to know their natures and causes, first superficially and then with increasing depth as a person's knowledge and experience are perfected. Then we ran up against the problem of human finitude and the fact that some objects of knowledge transcend our capacity for active learning.
The prime examples of this are the first cause of the existence of the universe, and the final goal of human desire (i.e., what would make us happy). These two things cannot be known by ordinary human knowledge. The first cannot be known because such a first cause could not be a material being (since then it too would be part of the universe and would demand explanation), and our faculties are suited only for the investigation of material things. The second frustrates our attempts to know it because every finite goal we can propose still leaves open the possibility of unhappiness — thereby demonstrating that the sine qua non of human fulfillment lies elsewhere. And yet it is practically necessary to admit the existence of a final goal for human desire, since without such a goal all human activity would be rendered meaningless. Likewise, we must suppose that there is some first cause of the existence of the universe. Everything manifestly exists and it is clear that each thing in particular is what it is because of some cause. There must, then, be a universal cause. (Cf. SCG I. chs. 10-14 or STh Ia q.2) [N.B., St. Thomas argues very effectively that the ultimate origin and end of things must both actually be the same being, whom we call "God".]
Oddly enough, people go around all the time claiming to know a great deal about both of these subjects. Frequently — especially on urban street corners — they are all too prepared to share their knowledge with others. How does this fit with what we have said about teaching?
Recall that the learner is instructed by having his gaze directed toward specific things so that his mind can act on them and come to know them. Knowledge, we said at the very beginning, is a kind of light which comes from the things known — not from the person teaching. How, then, could someone claim to teach us about things which transcend the human ability to know? How could such a person know them in the first place?
It is quite simply not possible. The human mind cannot reach up to the infinite and grasp it by its own power. If somehow we are to know the ultimate beginning and end of things, it follows that the intellect must be elevated above its ordinary nature in order to understand. Thus we speak of "supernatural knowledge". "Supernatural" here has little to do with spooks or magic tricks. Instead it highlights the fact that, given the limitations of the human intellect, in order to know an infinite object, the human capacity for knowing must be made adequate to its object. Furthermore, because our way of learning is limited to material things, such knowledge must be directly implanted into the mind. Here St. Thomas writes better than I could hope to, so I will simply quote him directly:
"Two things are requisite for faith. First, that the things which are to be believed are proposed to man: this is necessary in order that man believe anything explicitly. The second thing requisite for faith is the assent of the believer to the things proposed to him. Accordingly, as regards the first of these, faith must be from God. This is because those things which are believed by faith surpass human reason, hence they do not come to man's knowledge, unless God reveals them. To some, indeed, they are revealed by God immediately, as those things which were revealed to the apostles and prophets, while to some they are proposed by God in sending preachers of the faith, according to Romans 10:15, "How shall they preach unless they are sent?"
As regards the second, that is, man's assent to the things of faith, we can see a twofold cause: on one hand an external inducement, such as seeing a miracle, or being persuaded by someone to embrace the faith: neither of which is a sufficient cause, since some who see miracles or hear a sermon do not believe, while others do. Hence we must assert the existence of another, internal cause, which moves man inwardly to assent to matters of faith.
Now, the Pelagians held that this use was nothing else than man's free-will: and consequently they said that the beginning of faith is from ourselves, inasmuch as, to wit, it is in our power to be ready to assent to things which are of faith, but that the consummation of faith is from God, Who proposes to us the things we have to believe. But this is false, for, sine man, by assenting to matters of faith, is raised above his nature, this must accrue to him from some supernatural principle moving him inwardly; and this is God. Therefore faith, as regards the assent which is the chief act of faith, is from God moving man inwardly by grace." (from STh IIa IIae q.6 a.1)
Another quote, a little later in the text, is also illuminating:
"Understanding implies an intimate knowledge. This is clear to anyone who considers the difference between intellect and the senses, because sensitive knowledge is concerned with external sensible qualities, whereas understanding penetrates into the very essence of a thing, since the object of the intellect is what a thing is, as Aristotle says.
Now, there are many kinds of things that are hidden within, and to find them human knowledge must, so to speak, penetrate within them. Thus under the attributes of a thing is its substantial reality, under words lies hidden their meaning; under likenesses and figures the truth they denote lies hidden, and effects lie hidden in their causes, and vice versa.
Since, however, human knowledge begins with the outside of things as it were, it is evident that the stronger the light of the understanding, the further it can reach into the heart of things. Now the natural light of the understanding is of finite power; thus it can reach only to a certain fixed point. Consequently man needs a supernatural light, in order to penetrate further still so as to know what it cannot know by its natural light. And this supernatural light which is given to man is called the gift of understanding." (from STh IIa IIae q. 8 a. 1, with a few omissions and simplifications)
Faith, then is a kind of knowledge. It is supernatural in that the object of this knowledge transcends the human intellect, so that our minds must be both elevated and informed in order to have such knowledge. Both of these transformations can be accomplished by God alone, since it takes a being adequate to God to expand the intellect so as to know God, and there is nothing as infinite or as perfect as God, so that he himself must do this work in us. However, he uses instrumentally the work of teachers and preachers who are the mediating occasion for his work in us. Additionally, because the knowledge of faith pertains to God, who is infinite, it cannot be reduced to some short list of affirmations. At best it can be represented in summary form by a kind of creed or "symbol" which is used both to initiate people into the community of the faithful, and to draw the line between those who know the truth of faith and those who are in error about divine things.
But how can they call on him in whom they have not believed? And how can they believe in him of whom they have not heard? And how can they hear without someone to preach? And how can they preach unless they are sent? (Romans 10:14-15)
For by grace you have been saved through faith, and this is not from you; it is the gift of God; it is not from works, so no one may boast. For we are his handiwork, created in Christ Jesus for the good works that God has prepared in advance, that we should live in them. (Ephesians 2:8-10)