The real preaching is selling books. You should know the tactic how to sell without irritating. What your lecture will do for three minutes, but if he reads one page his life may be turned. We don't want to irritate anyone, however. If he goes away by your aggressive tactics, then you are nonsense and it is your failure. Neither you could sell a book, neither he would remain. But if he buys a book that is the real successful preaching.
Letter to Bali-Mardana, 30 September, 1972