1. Faith is an assent of the mind to a truth because God in his goodness has revealed it to us. The Will assents to something BEYOND the intellect's ability to grasp it.
2. As God--infinite, almighty, beyond what we can comprehend--is the object of Faith, this makes sense.
3. Therefore, an act of Faith is a reasonable thing, even as the object of that Faith is beyond reason's reach.
4. Reason needs Faith to preserve it from pridefully claiming to do what is beyond its reach, ie, proclaim on the inner life of God.