True Believers can't be Trusted?

Recently, a student explained to me that he was looking for sources on New Testament that were not connected to any seminary.  His professor had scoffed at the perspective of "insiders" and disallowed their comments from the academic discussion of the historicity of the NT.  This kind of reasoning is both preposterous and hopelessly prejudicial.  It would be akin to disallowing any work by an historian of Abraham Lincoln if the scholar believed what Lincoln said was true.  Or for that matter, no person of color would be allowed to comment on the history of Martin Luther King.  Why would one rule out any argument put forth by a believer?  The professor has merely shown his own cards that are prejudiced toward unbelief.  This is an unfair, unscholarly, and unseemly way to limit academic discourse.