By Fr. Dimitri Dudko
Religious fanaticism is a narrowness of vision, a blind confidence that you're right, an unwillingness to listen to someone else's opinion, impatience with others. If we don't fight religious fanaticism, it will grow into such a defect that it becomes a delusion, and this is a terrible thing. At the base of delusion lies inordinate pride, and (as we know), God opposes those who are proud (Jam. 4:6). Deliverance from delusion requires means which not all people possess. We must fight religious fanaticism, and our battle will be successful only when we depend not only on our own powers, but turn to humility and to the help of God. This will put us on guard against delusion.
But I'd like to mention that sometimes non-believers label anyone who believes a fanatic. This just goes to show that in this case it's the non-believer who's the fanatic, because he cannot tolerate any of the believers' arguments. Then sometimes they call any person with convictions a fanatic. Convictions aren't the same thing as fanaticism. Convictions are a person's confidence in what he does. Without this confidence a person has (as they say) no sense of direction. He's like a weather-vane that turns whichever way the wind is blowing. Thus, there are two extremes - religious fanaticism and lack of principle. Sometimes the latter is called "broad-mindedness" or "patience" by mistake.
From Our Hope (St. Vladimir's Seminary Press, 1977) pp. 43-44.