The importance of chanting Hare Kṛṣṇa, Hare Kṛṣṇa, Kṛṣṇa Kṛṣṇa, Hare Hare/ Hare Rama, Hare Rama, Rama Rama, Hare Hare is very strongly stressed in the Second Canto, First Chapter, verse 11, of Srimad-Bhagavatam in the following way. Sukadeva Gosvami tells Maharaja Pariksit, “My dear king, if one is spontaneously attached to the chanting of the Hare Kṛṣṇa maha-mantra, it is to be understood that he has attained the highest perfectional stage”. It is specifically mentioned that the karmis who are aspiring after the fruitive results of their activities, the Salvationists who are aspiring to become one with the Supreme Person, and the yogis who are aspiring after mystic perfections can achieve the results of all perfectional stages simply by chanting the maha-mantra. Sukadeva uses the word nirnitam, which means “it has already been decided”. He was a liberated soul and therefore could not accept anything which was not conclusive. So Sukadeva Gosvami especially stresses that it has already been concluded that one who has come to the stage of chanting the Hare Kṛṣṇa mantra with determination and steadiness must be considered to have already passed the trials of fruitive activities, mental speculation and mystic yoga.
The same thing is confirmed in the Adi Purana by Kṛṣṇa. While addressing Arjuna He says, “Anyone who is engaged in chanting My transcendental name must be considered to be always associating with Me. And I may tell you frankly that for such a devotee I become easily purchased”.
In the Padma Purana also it is stated, “The chanting of the Hare Kṛṣṇa mantra is present only on the lips of a person who has for many births worshiped Vasudeva”. It is further said in the Padma Purana, “There is no difference between the holy name of the Lord and the Lord Himself. As such, the holy name is not a material sound vibration, nor has it any material contamination”. The holy name cannot, therefore, be chanted offenselessly by one who has failed to purify his senses. In other words, materialistic senses cannot properly chant the holy names of the Hare Kṛṣṇa maha mantra. But by adopting this chanting process, one is given a chance to actually purify himself, so that he may very soon chant offenselessly.
Caitanya Mahaprabhu has recommended that everyone chant the Hare Kṛṣṇa mantra just to cleanse the dust from the heart. If the dust of the heart is cleansed away, then one can actually understand the importance of the holy name. For persons who are not inclined to clean the dust from their hearts and who want to keep things as they are, it is not possible to derive the transcendental result of chanting the Hare Kṛṣṇa mantra. One should, therefore, be encouraged to develop his service attitude toward the Lord, because this will help him to chant without any offense. And so, under the guidance of a spiritual master, the disciple is trained to render service and at the same time chant the Hare Kṛṣṇa mantra. As soon as one develops his spontaneous service attitude, he can immediately understand the transcendental nature of the holy names of the maha-mantra.
Source: A.C. Bhaktivedanta Swami Prabhupada (2011 edition), “The Nector of Devotion”, Page 106