Allah says, “And if you obey most of those in the earth, they will lead you astray from Allah’s way. They follow nothing but conjecture and they only lie.” [6:116]
Conjecture, translated from dhann, is baseless opinion, assumptions, basically empty thought that is not grounded in truth, reality, and understanding. Allah tells us very clearly that most people live and breath on the basis of dhann. But they incorrectly believe that they are on solid ground, so much so that they try to lead people. In other words, they are confident and speak with authority and act like they know what they are talking about it. In this way, they try to get people to follow them in their error.
But the Muslim has to be skeptical of these false leaders. Even if a person speaks with authority or has credentials or uses charming, even persuasive language, be careful. Judge on the basis of the manifest, undoubted truths of Islam. More likely than not, such people are only speaking from dhann.
As an example, every few years a new scientific study comes out on the “health benefits” of drinking alcohol. These results are touted and secular, colonized Desis, Arabs, Persians, etc., point to the studies as proof of how backwards Islam is. But inevitably, all these studies turn out to be flawed and overturned or the results turn out to be ambiguous and ultimately un-repeatable and baseless.