Proverbs 18:1-2 is difficult to translate but shows us an important truth.

Through desire a man, having separated himself, seeketh and intermeddleth with all wisdom.  KJV

A man who isolates himself seeks his own desire; He rages against all wise judgment. New King James Version

Holding on  to a false doctrine while claiming to be a fundamentalist, a Christian can find himself constantly being defeated by wisdom (the ability to apply knowledge to a specific situation). So he starts preaching isolation, while labeling it separation. You cannot listen to Billy Graham! or the Southern Baptist Convention! or Jerry Falwell! or Bob Jones University! or the charismatics! or Adam Clarke! or Strong's Exhaustive Concordance! or anyone showing the sins of the leader's group!

Eventually, the leader isolates his group enough that his followers cannot learn the errors of their leader.