Prabhupada's lecture - Śrīmad-Bhāgavatam 1.5.36 - Vṛndāvana, August 17, 1974
Another point is as soon as you try to become Kṛṣṇa conscious, then your purification begins immediately. Śṛṇvatāṁ sva-kathāḥ kṛṣṇaḥ puṇya-śravaṇa-kīrtanaḥ [SB 1.2.17]. If you simply remember or say that "I have heard it from my spiritual master or from a saintly person, that while drinking water, that taste is Kṛṣṇa. So you accept it," this is preaching. This is preaching. You are preaching. Or one who hears from you, śṛṇvatāṁ sva-kathāḥ, if you simply carry this work. Just try to understand the importance of preaching. Sva-kathāḥ kṛṣṇaḥ, you are remembering and you are preaching. So you are preaching means somebody else is hearing you. So he is also being purified. Śṛṇvatāṁ sva-kathāḥ kṛṣṇaḥ puṇya-śravaṇa-kīrtanaḥ [SB 1.2.17]. Even he does not know what is Kṛṣṇa, but he simply hears from you, then he becomes purified.
Actually, we are experiencing how in European and American countries, our boys going in buses, village to village, and chanting Hare Kṛṣṇa mantra, and they are collecting devotees. [break] How many devotees collected you told me, France? Ten. Where? You sent your saṅkīrtana party there? It is happening. Our saṅkīrtana movement is increasing in every country. Europe, America, even in Africa, Canada, everywhere. Why? Caitanya Mahāprabhu, when He was traveling in South India, He was chanting Hare Kṛṣṇa mantra. So somebody heard Him chanting this mantra, he became purified, and he went back to his village, he also began to chant. So another person who heard him, he became purified. It is such a nice thing.