Faith is the substance of what we hope for, the evidence of what we cannot see, says the letter to the Hebrews. Faith is a disposition we characterize by the activity of believing it disposes us to, and that activity we characterize by the object believed in. Now believing is an assent of mind commanded by the will, so faith's activity relates to its object both as to a good willingly pursued and as to a truth mentally assented to. Moreover, being a theological virtue in which goal and object are identical, the way the object is faith's goal will correspond to the way it is its object. Now faith's object is unseen Truth itself and whatever else we assent to because of that Truth. So faith's goal is also Truth itself as unseen, that is to say unachieved yet hoped for. So Hebrews expresses the way faith relates to Truth as goal, the object willed, by saying Faith is the substance, or seed, of what we hope for, since what we hope for is to see openly the Truth we already assent to by faith.
From: Summa Theologiae
A Concise Translation
Timothy McDermott