One of my New Year's resolutions is to read Peter Kreeft's Summa of the Summa. It is an abridged version of Thomas Aquinas's Summa Theologica, annotated by a Boston College professor.
Here's my summary Thomas's argument concerning the nature and extent of Sacred Doctrine in the first chapter:
Scripture is the starting point for the study of theology because it has been revealed by God. We know it has been revealed by God because it says it has. We can't use reason to prove what has been revealed by God; indeed, we have no need to. But reason may be used to refute arguments which object to revealed truth.