A spirit ushered by false teachings is like a body nourished by sweets. The adult, as opposed to the child, will come to understand reasons one cannot live a healthy existence on nothing but candy; so likewise, the Christian must come to understand that one cannot know and love the will of God under false doctrine.
So many people concentrate on knowing who a good preacher or a bad preacher is and so many people concentrate on knowing what the word of God is! The former is good but the latter is noble. The latter is the best answer to the former. The real duty of man is never to know man but to know God through Christ Jesus! Knowing the word of God is the true way to knowing God!
Heresy would like to think of itself as 'invented Truth'. But of course, all Reason and Logic would agree that no man can ever create Truth; he can only discover it. If heresy were ever at all beneficial, God would use it really to bring one right back to Truth, as countless 'inventions' have brought men to discovery.
What we fail to realize is we often become like Pharisees in our ruthless attempts to identify Pharisees (and impostors). While indeed some people use the old laws of religious pride to tear down men of God, others use the new laws of anti-religious anger to tear down men of God.
False prophet: A prophet whose god is not yet known
It seems to me that those who claim to be 'true' and 'bible believing' Christians and yet spend all their time and energy spreading unhappiness and hatred for others - and are themselves consumed by it - are by the very definition of the term - false prophets.