Perhaps you have experienced one God called preacher condemning another as a false teacher. I certainly have. The associated irony did not always register on my brain.
Absolutely nothing identifies anyone as a rightful instructor of others in the requirements of God other than their own decision to claim that role. Naturally, preachers who condemn others as false teachers will say that the Bible validates their instruction and therefore corroborates their legitimacy as proper instructors.
If, in fact, the Bible is the written standard by which I am able to vet all would be instructors, then I don't need an instructor at all. I can just read the instructions for myself.
If the instructors assert that the ordinary man cannot properly interpret the Bible but must have extraordinary human assistance, then I end up right back where I started, needing a way to identify a valid instructor but having none. Either the words of the Bible are the standard or the words of the preacher; which is it?
The bottom line should be obvious. All any preacher can offer is his personal take on the Bible or more likely just a regurgitation of the traditional interpretation. Realistically, until God speaks directly in each and every ear, the divine truth will remain guess work, no matter how long a particular version may have been taught and held sacred.
The history of the institutional church, with all its schisms and theological arguments and revisions, amply demonstrates the many ways various groups have interpreted and re-interpreted the Bible. Within that context it is preposterous to the extreme to claim to be the bona fide Bible instructor and condemn all others who disagree.