This passage is a famous quote by John Stuart Mill from his book On Liberty. On Liberty is one of the most remarkable defenses of freedom of speech in political thought.
Mill begins by telling us that many people hold opinions for which they have no facts or grounds to defend them, even against superficial objections. Therefore, these people are completely convinced of certain beliefs they in fact should not believe.
Moreover, he believes that these people generally think it a bad thing to question those beliefs, as they were taught by an authority who they consider more knowledgeable than themselves. Therefore, they are incapable of rejecting an opinion in a well-thought out way (as they do not usually consider things wisely), and instead, when they reject an idea they do so in a rash way.
This people might be mistaken in their opinions, or they might be correct. However, even when they are correct, this should not be called "truth." A "truth" that is known in this way is simply another type of prejudice or superstition, as the process through which it was obtained was not one of honest and open dialogue.