No one knows the hearts of all men. My experience has been that most of the ones whom I have been able to study with are sincere. If you ask me “Do they study their Bibles?” Once more I find that those whom we are able to study with do indeed, study their Bibles. The problem is, in our times, people tend to not put a good deal of effort into that study. The followers of Jesus are commanded to study in order to rightly divide the Word of truth (2 Timothy 2:15). One cannot read the Bible either casually or in an effort to find a loophole and expect to have good results.
People can attend gatherings where there are large audiences and hear some preacher tell them what the Scriptures are saying. I have seen via television some in those audiences with their Bibles opened and some even making notes. They trust what the preacher says and, therefore, their understanding is colored by what he believes. Paul taught to not think of men above that which is written (I Corinthians 4:6). When we hear others preaching and or teaching our job is to make an evaluation if what we have heard is true or not (Acts 17:11). My experience has been that most individuals do not make that effort but trust what the preacher told them.
They hear a doctrine and believe it. In time, they pass it down to their children and so on down the line. Each step of such a journey should say that such individuals are sincere. Sincerity, however, is never the standard by which we are saved. Naaman thought the idea of dipping seven times in Jordan was beneath him. A servant reasoned with him and he went to the Jordan and dipped seven times (II Kings 5:1-14). Was he sincere prior to dipping in the Jordan? Yes, but if he had stayed sincere, he would have died of leprosy. His sincerity led him to obey what Elisha had told him to do. Sincerity without action is never enough.