The New Testament is clear about my relationship with evil (Ephesians 5:11). When I give encouragement to the man who teaches contrary to the doctrine of Christ, I become a partaker with him in his evil works (Romans 16:17-18; 2 John 9-11).
It is an evil work to teach error and it is likewise an evil work to support, sustain, encourage or have fellowship with a teacher of error. Sometimes it becomes hard for us to refuse friends of long-standing that go off after error and accept erroneous doctrines and instead of abiding in the doctrine of Christ. We allow our sympathy, affection or esteem to get the better of us and we refuse to draw the line of fellowship when the truth has been violated and error is being taught.
I am not being extreme; I am following the scriptures.