Complex sentence: The core of the book examines how the Pauline epistles were read by 2nd century Valentinian gnostics and hypotizes that Paul could be considered a proto-gnostic as well as a proto-Catholic.
Split into simpler sentences: The core of the book examines how the Pauline epistles were read by 2nd century Valentinian gnostics. That is how Paul could be considered a proto-gnostic as well as a proto-Catholic.