Why do people end up misled after listening to preaching that isn't in the Bible?

Les warns that a message can come from a pulpit yet not be found in the Bible. He says if preaching lines up with the Book, fine, but much of what is heard is not in the Book; it is men's ideas. People swallow it hook, line, and sinker and then suddenly realize they have been led down some back alley. He connects this to Paul's statement that if our gospel be hid, it is hid to them that are lost. The better translation is veiled. As in chapter 3, the veil is upon the heart, and when the heart turns to the Lord, the veil is taken away. So the problem is both preaching that replaces Scripture with men's ideas and a veil that keeps lost hearts from comprehending the gospel until they turn to the Lord.