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When someone heard the chanting of the holy name from the mouth of Śrī Caitanya Mahāprabhu, and someone else heard this chanting from that second person, and someone again heard this chanting from the third person, everyone in all countries became a Vaiṣṇava through such disciplic succession. Thus everyone chanted the holy name of Kṛṣṇa and Hari, and they danced, cried and smiled. (Madhya-lila 17. 48-49)
The transcendental power or potency of the Hare Kṛṣṇa mahā-mantra is herein explained. First, the holy name is vibrated by Śrī Caitanya Mahāprabhu. When someone hears from Him directly, he is purified. When another person hears from that person, he also is purified. In this way the purification process is advanced among pure devotees. Śrī Caitanya Mahāprabhu is the Supreme Personality of Godhead, and no one can claim His potency. Nonetheless, if one is a pure devotee, hundreds and thousands of men can be purified by his vibration. This potency is within every living being, provided he chants the Hare Kṛṣṇa mahā-mantra offenselessly and without material motives. When a pure devotee chants offenselessly, another person will become a Vaiṣṇava, and from him another Vaiṣṇava will emerge. This is the paramparā system. (Purport)
The Hare Krishna Movement 