Preaching requires strength. Preaching also requires faith. Without faith and the subsequent strength that one gets from surrendering, it is very difficult to go on preaching. As we mature in Krishna consciousness, it becomes more and more clear that preaching cannot continue on the external platform. Preaching is not an external engagement, simply accumulating scores and looking at results from an external point of view. If we want to be successful preachers, there must be an internal life as well. This will give us the conviction and strength required to distribute Krishna consciousness because we, ourselves, know the real value of Krishna consciousness.
(Collected Letters to My Disciples, Vol 3, Pg 106 2nd Paragraph)