For a preacher there are four principles to be followed. One, he must be fully surrendered to Krishna. Two, intimately in friendly relationship with devotees trying to elevate conditioned living entities to Krishna Conciousness and rejecting the nondevotee class. So if you find a nondevotee eager to listen submissively you can show your mercy to elevate him to Krishna Conciousness, but a nondevotee who is already poisoned by the serpent of the demon class swami or yogi is very difficult to be dealt with, so the best thing is to reject them and not waste time for their elevation.
Letter to Gurudasa, 24 February, 1968