“At Glaurus he began to realize that he, like most other clerics, had been accustomed to pay far too much attention to externals and to Bible commentaries instead of to the text itself.” – Potter, 39
A master of the secondary literature is not necessarily a master of the primary literature. It is the primary literature you must master if you are to understand a subject. The problem is that only those who have some mastery of the primary literature can judge whether another has mastered it. What mastering the secondary literature gives you is access to exactly that: the secondary literature. It is an illusion to think you have mastered the primary simply because you have mastered the secondary. It is an illusion anybody can maintain as long as there are few who have mastered the primary texts. But there are no alternatives to dealing with the primary material. And it is the same when it comes to the Bible and its secondary literature, commentaries.