Fanaticism is never truly spiritual because it is not free. It is not free because it is not enlightened. It cannot judge between good and evil, truth and falsity, because it is blinded by prejudice. Faith and prejudice have a common need to rely on authority and in this they can sometimes be confused by one who does not understand their true nature. But faith rests on the authority of love while prejudice rests on the pseudo-authority of hatred. Everyone who has read the Gospel realizes that in order to be a Christian one must give up being a fanatic, because Christianity is love. Love and fanaticism are incompatible. Fanaticism thrives on aggression. It is destructive, revengeful and sterile. Fanaticism is all the more virulent in proportion as it springs from inability to love, from incapacity to reciprocate human understanding.
Thomas Merton, Disputed Questions, 1953/88, p132