The devotees and the Lord are interlinked, and they cannot be separated. Therefore talks about them are all kṛṣṇa-kathā, or topics of the Lord.
(Srimad Bhagavatam----1:7:12----purport).

Transcendental topics of the activities of Lord Śrī Kṛṣṇa in the Śrīmad-Bhāgavatam begin from the end of the battle at Kurukṣetra, where the Lord Himself spoke about Himself in the Bhagavad-gītā. Therefore, both the Bhagavad-gītā and Śrīmad-Bhāgavatam are transcendental topics of Lord Kṛṣṇa. The Gītā is kṛṣṇa-kathā, or topics of Kṛṣṇa, because it is spoken by the Lord, and the Bhāgavatam is also kṛṣṇa-kathā because it is spoken about the Lord. Lord Śrī Caitanya Mahāprabhu wanted everyone to be informed of both kṛṣṇa-kathās by His order. Lord Kṛṣṇa Caitanya is Kṛṣṇa Himself in the garb of a devotee of Kṛṣṇa, and therefore the versions of both Lord Kṛṣṇa and Śrī Kṛṣṇa Caitanya Mahāprabhu are identical. Lord Caitanya desired that all who are born in India seriously understand such kṛṣṇa-kathās and then after full realization preach the transcendental message to everyone in all parts of the world. That will bring about the desired peace and prosperity of the stricken world.
(Srimad Bhagavatam----1:7:13-14----purport).

The activities of Kṛṣṇa are nondifferent from Kṛṣṇa Himself. As long as one is engaged in hearing such transcendental activities of Kṛṣṇa, he remains aloof from the conditional life of material existence. The topics of Lord Kṛṣṇa are so auspicious that they purify the speaker, the hearer and the inquirer. They are compared to the Ganges waters, which flow from the toe of Lord Kṛṣṇa. Wherever the Ganges waters go, they purify the land and the person who bathes in them. Similarly, kṛṣṇa-kathā, or the topics of Kṛṣṇa, are so pure that wherever they are spoken, the place, the hearer, the inquirer, the speaker and all concerned become purified.
(Srimad Bhagavatam----2:1:1----purport).

There are two important kṛṣṇa-kathās current in the world—Bhagavad-gītā and Śrīmad-Bhāgavatam. Bhagavad-gītā is kṛṣṇa-kathā because it is spoken by Kṛṣṇa, whereas Śrīmad-Bhāgavatam is kṛṣṇa-kathā because it narrates about Kṛṣṇa. Lord Caitanya advised all His disciples to preach kṛṣṇa-kathā all over the world without discrimination because the transcendental value of kṛṣṇa-kathā can purify one and all from material contamination.
(Srimad Bhagavatam----3:5:10----purport).

Kṛṣṇa-kathā is so powerful that simply by entering into a person's ear it can at once give deliverance from the bondage of family affection. Family affection is an illusory manifestation of the external energy, and it is the only impetus for all mundane activities. As long as there is mundane activity and the mind is absorbed in such engagement, one has to undergo the repetition of birth and death in the current material nescience. People are most influenced by the mode of ignorance, and some are influenced by the passionate mode of material nature, and under the spell of these two modes a living being is actuated by the material conception of life. The mundane qualities do not allow a living entity to understand his real position. The qualities of both ignorance and passion strongly bind one to the illusory bodily conception of the self. The best among the fools who are thus deluded are those who engage in altruistic activities under the spell of the material mode of passion.
(Srimad Bhagavatam----3:5:11----purport).