God lamented in Jeremiah that the religious leaders had led His people astray. 2 Timothy 3 addresses this problem in the lives of Christians and says in part,
"But evil men and impostors will proceed from bad to worse, deceiving and being deceived. You, however, continue in the things you have learned and become convinced of...All Scripture is inspired by God and profitable for teaching, for reproof, for correction, for training in righteousness; so that the man of God may be adequate, equipped for every good work."
Christians can't "just trust" the preacher, but there are very FUNCTIONAL reasons for that beyond just hysterical paranoia. Getting very familiar with God's word is what equips us for "every good work," and part of that good work is sometimes pointing out Satan's deception.
So let's get to know the word for ourselves and become stronger warriors! :)
"For the entire Law is fulfilled in in this one word: "You shall love your neighbor as yourself."—Gal 5:14