I've heard that faith is a theological virtue. What does that mean?
To say a virtue is theological means that it comes as a gift from God. St. Thomas Aquinas, a Dominican monk who crafted the famous arguments for the existence of God, wrote: "This alone is the true knowledge of God: to know that God is beyond knowing." God is transcendent. This word comes from the Latin "transcendere," meaning "to climb beyond." When something is transcendent, it exists beyond our normal range of experience. Philosopher Peter Kreeft put it simply: "God is always more."
While God created us in His image and likeness, too often we re-create Him in ours. We project our deepest hopes, our subliminal fears, and our hidden dreams, and conjure up our own image of what God could be. Yet God can never be limited to our own perception of Him.
That's where faith comes in. We read in the Catechism that "faith is the theological virtue by which we believe in God and believe all that He has said and revealed to us and all that the Holy Church proposes for our belief" (CCC 1814). We can know God through reason, to a point. As human beings, we want to know and understand the world around us, but God isn't in the world around us. He's beyond it. The best way to come to know God is not through our own clever mental maneuvering, but through trusting in God's revelation of Himself. We might think we know who God is, but the only way we can have any idea is if we trust, through the virtue of faith, who He has revealed Himself to be.