Unless one is a first-class devotee, he cannot be a preacher. A preacher is generally a topmost devotee, but in order to meet the general populace, he has to come to distinguish between devotees and nondevotees. Otherwise, an advanced devotee makes no such distinctions. Indeed, he always sees that everyone is engaged in the service of the Lord. When one engages in preaching work, he must distinguish between people and understand that some people are not engaged in the devotional service of the Lord. The preacher then has to take compassion upon such innocent people who do not know how to worship the Lord. In Śrīmad-Bhāgavatam (11.2.45), the symptoms of a topmost devotee are described as follows:

sarva-bhūteṣu yaḥ paśyed bhagavad-bhāvam ātmanaḥ
bhūtāni bhagavaty ātmany eṣa bhāgavatottamaḥ

“The advanced devotee sees that all living entities are part and parcel of the Supreme Personality of Godhead. Everyone is in Kṛṣṇa, and Kṛṣṇa is also within everyone. Such a vision is possible only for a person who is very advanced in devotional service.”