There are four things for a preacher

For a preacher, there are four things:

īśvare tad-adhīneṣu
bāliśeṣu dviṣatsu ca
prema-maitrī-kṛpopekṣā
yaḥ karoti sa madhyamaḥ -SB 11.2.46

When in the lower stage of devotional service, he cannot become preacher. When he's in a little upper, second stage, he can become preacher. So preacher has to see four things. First of all God, īśvara, and tad-adhīneṣu, and those who are devotee. God, His devotees, and bāliśa, innocent, he does not know anything about... So three = God, devotee, and the innocent. And dviṣat, and envious, atheist class. He has to see four things, and he has to deal with four persons differently.

1. With God, īśvare prema: how to advance my love for God, these dealings. 
2. Prema-maitrī: and to the devotees, we have to make friendship with them. Prema-maitrī. 
3. And to the innocent, we have to preach, kṛpā: "Oh, here is an innocent person. He does not know; he's eager to learn." There teaching is required. Teaching, you cannot teach God or you cannot teach God's devotee. But you can teach only the innocent. 
4. And those who are dviṣat, atheist, upekṣā, don't go there, save yourself. These are the four things. So when one is not open to hear, then don't bother yourself. That requires very strong preacher to convince the atheist class, provided he is reasonable also. If he's stubborn, obstinate, then it is also very difficult. 

But preaching is meant, innocent; that one who is actually sincere but he does not know what is God, what is my relationship with God, there is necessity of preaching. Not to the envious or those who are already advanced, or to God.

- Lecture July 5 1976, Washington DC, Śrī Caitanya-caritāmṛta, Madhya-līlā 20.100 by Srila Prabhupada