The world is divided into two types of people: Those who test their own beliefs and those who get quite cross if you attempt to encourage them to do so. In Acts 17, we see the perfect picture of both. Paul spent three Sabbaths preaching the Gospel in Thessalonica, but his preaching angered the Jews, who mobbed the house he was staying at. Paul fled to Berea, instead preaching the Gospel there. Here, Paul found a more hospitable environment. The people, rather than attacking him, listened to what he said, and more importantly, they tested what he was saying. They then believed him.
A key attribute of truth is that it can withstand itself. If you throw honest tests at an honest worldview, it will not fall down. This resulted in those who tested Paul's words in Berea were called "fair minded".
False worldviews are naturally resistant to honest testing. They cannot withstand honest tests, but they can disarm or replace them. Take, for example, the Watchtower Organisation. Jehovah's Witnesses are taught to be very skeptical of any source that doesn't have the Watchtower seal of approval, and according to the Watchtower, "...the Bible cannot be properly understood without Jehovah’s visible organization in mind." This is because they believe the Bible is "an organizational book and belongs to the Christian congregation as an organization, not to individuals, regardless of how sincerely they may believe that they can interpret the Bible."
Already you can see the problem. Whereas the Watchtower teaches that even the most sincere of individuals cannot correctly interpret the Bible, the Jews in Berea were called "fair minded" for doing just that, even though Paul was the messenger they were testing. It was the Jews in Thessalonica who believed, in their envy, that only their (blatantly false) interpretation of the Scriptures was authoritative, who stand condemned before the Lord.
This teaching could not be more vital. The only defence against false teachers is to honestly study the word of God on a regular, preferably daily basis, and so it is the practice of such false teachers to claim that you cannot do that. But you can. And you must. Not even the Apostles would have dared tell you not to try to study the Scriptures alone, and so if anyone ever does, that is an immediate alarm bell. I would like to go on record right now and say that I beg you not to take anything I say at face value. I have made errors in the past. Though I obviously do not know where they are (if I did, I would obviously change them), I hold errors now. I will continue to make errors in the future. In order to defend yourself against my errors, test my claims against the word of God, because where I make errors, the Lord does not. Therefore, rely on His counsel, and always test those who claim to speak in His name against His written word.