Srila Prabhupada declared the war against Kali. Kali is well equipped, he has a lot of weapons. We have two weapons – "Srimad-Bhagavatam" and "Bhagavad-gita", and these are the most powerful weapons.
The parampara system is such that somebody who is preaching has to realize the message he is preaching, otherwise it will be just an exploitation of the innocent people. Realization means reading Srila Prabhupada’s books and living these books. We also have to hear very attentively, with eagerness.
Our mind always wants to do something else, does not matter what he is doing now, he is always in the future, therefore he is inattentive. When you preach you mind becomes attentive. First we have to train ourselves be attentive when we hear, or when we read. If we hear with attentiveness and concentration, the realization will come. When we hear the pure message of "Srimad-Bhagavatam" it goes deep into our ear, but we can have some blockage, therefore the process of hearing with a proper attention purifies our ears.
If we hear with a proper attention then we will be enthusiastic. Our enthusiasm is coming from sraddha. If we want to preach we should be realized, otherwise it’s just formality and people will not be changed after our preaching, because the purpose of preaching is to try to help another person to change the heart.
Without hearing we can not preach. If we hear with rapt attention, whatever we hear it becomes realized and when we speak the other people will be profoundly influenced. The main principle of preaching is that you do what you preach yourself.