We should seek simplicity,
but in that very search
we should be aware of
the many illusions and delusions.
I know that at the core
of each religion
there is truth,
heavily obscured in some cases,
but it is there
--- Echkart Tolle
There is a whole branch of philosophy concerned with the nature and
limitations of knowledge called Epistemology. It addresses questions
like how knowledge is acquired, distinguishing between belief and truth
and being skeptical of anything that is not known to be truth.
Years have been spent studying the subject and it seems the only reliable way to "know" something (that all can pretty much agree upon) is by directly experiencing it through the use of our five senses. Also, it must be continually repeatable. If someone "knows" something, then the next person should be able to directly "know" it as well using their five senses.
If someone says they "know" something and we cannot recreate the experience to directly "know" it ourselves, then we are justified in being skeptical and very possibly are dealing with someone's beliefs or faith or trust or intuition which is not knowing.
Beliefs are usually based on the words of a person or group or school of thought we consider to be an authority. We take up these beliefs because they appeal to us and over time, we begin to believe they are true. It is reasonable for us to believe something is true while leaving the door open that it may be false. However, it is an error to for us to say we "know" unless we have actually experienced it.
As we can see, not only individuals tend to think they "know" something when they really only believe something, delusions of "knowing" can be more popular and can spread to crowds.