For there must be also heresies among you, that they which are approved may be made manifest among you. Per 1 Corinthians 11:19.
Those with errors, eventually get exposed and are found to believe other errors. So, you can tell someone is either immature, or a backslider or fake. Most of Churchianity is false. The Reformation was ONLY a tiny start. But thanks to the printing press it was a start. Before that, power politics ruled and errors blossomed. Now with the internet, another growth spurt has occurred. To whom much is given, much is required says the Gospel of Luke. The old heresies are falling away, one by one.