Credo ut intelligam—I believe in order that I may know. While St. Anselm, following St. Augustine, said this in speaking of Christian faith, what is articulated in this tight Latin turn of phrase does not apply only to the truths of the Christian faith. Rather, it highlights that no knowledge is ever fully available to the one who does not trust. This is why we can speak not simply of faith as a theological virtue but also of natural faith. As John Henry Newman pointed out, if we do not trust our senses or our intellects, we cannot even begin the process of knowing. Everything we know—or even think we know—we know by some combination of faith and reason.
It is not that, as is perhaps easier to imagine, we know certain truths by faith (for example, that God became one of us in Jesus Christ) and others by reason (e.g., that the earth orbits the sun rather than vice versa). Faith and reason both play a role in each of these kinds of knowing—and in any kind of knowing at all.
Christian faith is not, as is sometimes asserted, blind. From the beginning, Christians have presented evidence for their claims—the Resurrection of Jesus first and foremost. Consideration of this evidence might lead a person to believe or to doubt, but evidence is offered. No one is invited to believe without reference to some argument that belief is reasonable. But evidence does not compel belief. Instead, it makes the choice of where to put one’s faith clearer. Reason clears the ground for faith.