It's quite trusting to believe anything that comes out of someone's mouth, but to say that applies to most of human knowledge just isn't right, at least in First World countries where most people have a basic education. Most of our knowledge comes from multiple individuals who have researched a subject. It may not always be up to date (textbooks are usually behind) or "the truth"(science and literary investigation are far from perfect), but it's not "trusting" to assume that a large part of the research others have done is valid and worth taking into consideration, particularly since a responsible individual acquires information from more than one source before drawing conclusions.

However, I would say that the opposite of your statement (disregarding any information that is not acquired by one's self) is paranoid.