Though the term "faith" exists even in our increasingly secular age, it is arguably a word misunderstood by many who use it. Take a moment to consider your own response to the question, posed perhaps by a non-Christian seeking after the truth: "What is faith?" Is it an opinion you have about God, the world, or human beings? If so, it is not faith. Is it something you hold to be true from time to time, albeit with the dread that necessarily accompanies doubt? If so, it is not faith.
I have presented the bad news first. Many have misconceptions about the nature of faith. This is bad news because faith is vital for us as human beings destined to be with God forever. Think of our Lord's words in Mark 1:15, "The time is fulfilled, and the kingdom of God is at hand; repent, and believe in the gospel." Belief, then, faith, is the proper response to the coming of God's kingdom. Without faith, as Hebrews 11:6 tells us, it is impossible to please God. If the bad news is that there are not a few misconceptions about faith, the good news is this: Faith is far richer and more glorious than our misconceptions of it (and indeed, even than our accurate notions).
Let us start with what we might call "natural faith." To believe is an act of the mind, and we perform this act frequently. Imagine that I approach you and reveal my name to you. In this act of self-disclosure, I have communicated to you a certain truth, namely, the truth about my name. Unless you have some sufficiently grave reason to suspect the falsity of my self-disclosure (perhaps I am a notorious liar or a lunatic), you assent to the truth of my words. Notice, you assent not because you know my name. You assent because your will moves your intellect to do so, in view of some good (the good of friendship, say). We perform this kind of mental act all the time. The reality of our belonging to this particular male and this particular female (our parents) is something largely a matter of faith, of believing. Now, it is true that the will does not always move our intellect to assent for the sake of a legitimate good. The persons who raised us could be deceiving us, after all. Even still, the act of believing is (or can be) rationally justified. The legitimacy of the assent is proportionate to the trustworthiness of the person communicating something to us.
Supernatural faith is something like natural faith. It is, to use St. Augustine's definition, "to think with assent." There is a significant difference, however, for supernatural faith is given to us by God, by the One who can neither deceive nor be deceived. Faith is a "theological" virtue because it is a divine gift. It is something that radically exceeds our natural human capacity to know. Just as grace itself does not destroy nature, but rather elevates and perfects it, so too does faith elevate and perfect the intellect. But faith is also called theological because its object is God. Faith, as an act of assent to the God who is Truth, is made to be the kind of act it is by God Himself.
We are given something of a definition of faith in Hebrews 11:1, "Now faith is the substance of things to be hoped for, the evidence of things that appear not." The sacred author is speaking here of what is often called living or formed faith, the saving faith mentioned by St. Paul in Galatians 5:6, "For in Christ Jesus neither circumcision nor uncircumcision is of any avail, but faith working through love." It is not the barren, lifeless faith described by St. James in chapter two of his epistle, a faith that even the demons possess. The main point here in Hebrews is that faith, supernatural faith that is animated by charity, is the beginning of eternal life in us. It is for this reason, at least according to theologians such as St. Thomas Aquinas, that the sacred author calls faith a "substance." We have a foretaste and down payment, even in our wayfaring state, of the life of heaven. In faith, the "things to be hoped for," namely, intimate knowledge of God, the life of heaven, supernatural communion with the Divine Persons, come to be in us. Faith is that whereby the knowledge we will have in heavenly glory comes to be in us, albeit in a veiled and incipient way. Faith, then, the first of the theological virtues, is the beginning of eternal life and the principle of drawing near to God.
One may wonder what the term "evidence" means in Hebrews 11:1. Faith is in some manner "the evidence of things that appear not." St. Thomas says that "evidence" here means the result of evidence. In other words, just as evidence leads us to adhere firmly to some truth, so does God move us to adhere firmly to His revealed truth. Notice that this firmness distinguishes faith from acts of the mind such as opinion, suspicion, and doubt. But because the firm assent concerns "things that appear not," faith is distinguished from knowledge and understanding, since for these latter two we need something apparent. Far from being contrary to reason, faith is super or supra-rational. It transcends the whole order of natural reason even if it does not contradict or undermine it; the higher does not negate the lower. St. Thomas says that the "definition" of Hebrews 11:1 amounts to this: faith is a habit of the mind, whereby eternal life is begun in us, making the intellect assent to what is non-apparent (ST II-II, q. 4, a. 1, resp.).
How often faith is misunderstood! How often we hear faith spoken of as a feeling, as an opinion to be discarded when it becomes inconvenient, as a Kierkegaardian leap with a dubious relationship to truth and reality. On the contrary, supernatural faith is an assent to the God who reveals, the God who wills in heavenly glory fully to manifest Himself to us. It is the beginning of eternal life even now in this vale of tears. Let us pray, then, for the gift of faith, that we may know God even as He knows Himself, and consequently hope in His promises and love Him fervently.
Thanks for the comment, Maria. May God increase our faith!