Examine the Scriptures
Gal 4:21
Tell me, you who want to be under the law, are you not aware of what the law says?
The gullible uncritically believe whatever they hear from leaders they trust without examining the source materials themselves. Those who have been denominationally indoctrinated often exhibit this weakness. But even concerning Paul's teachings the Bereans were commended for their skepticism in examining the scriptures every day to confirm whether what Paul said was scriptural.
Furthermore the gullible often don't consider the implications of their faith, since they lazily rely upon others to think for them. Examine your beliefs and consider whether there are implications you may have overlooked.