SB 6.2.12

naikāntikaṁ tad dhi kṛte ’pi niṣkṛte
 manaḥ punar dhāvati ced asat-pathe
tat karma-nirhāram abhīpsatāṁ harer
 guṇānuvādaḥ khalu sattva-bhāvanaḥ
Synonyms: 
na — not; aikāntikam — absolutely cleansed; tat — the heart; hi — because; kṛte — very nicely performed; api — although; niṣkṛte — atonement; manaḥ — the mind; punaḥ — again; dhāvati — runs; cet — if; asat-pathe — on the path of material activities; tat — therefore; karma-nirhāram — cessation of the fruitive reactions of material activities; abhīpsatām — for those who seriously want; hareḥ — of the Supreme Personality of Godhead; guṇa-anuvādaḥ — constant chanting of the glories; khalu — indeed; sattva-bhāvanaḥ — actually purifying one’s existence.
Translation: 
The ritualistic ceremonies of atonement recommended in the religious scriptures are insufficient to cleanse the heart absolutely because after atonement one’s mind again runs toward material activities. Consequently, for one who wants liberation from the fruitive reactions of material activities, the chanting of the Hare Kṛṣṇa mantra, or glorification of the name, fame and pastimes of the Lord, is recommended as the most perfect process of atonement because such chanting eradicates the dirt from one’s heart completely.
Purport: 

The statements in this verse have been confirmed previously in Śrīmad-Bhāgavatam (1.2.17):

śṛṇvatāṁ sva-kathāḥ kṛṣṇaḥ
 puṇya-śravaṇa-kīrtanaḥ
hṛdy antaḥ-stho hy abhadrāṇi
 vidhunoti suhṛt satām

“Śrī Kṛṣṇa, the Personality of Godhead, who is the Paramātmā [Supersoul] in everyone’s heart and the benefactor of the truthful devotee, cleanses desire for material enjoyment from the heart of the devotee who relishes His messages, which are in themselves virtuous when properly heard and chanted.” It is the special mercy of the Supreme Lord that as soon as He knows that one is glorifying His name, fame and attributes, He personally helps cleanse the dirt from one’s heart. Therefore simply by such glorification one not only becomes purified, but also achieves the results of pious activities (puṇya-śravaṇa-kīrtana). Puṇya-śravaṇa-kīrtana refers to the process of devotional service. Even if one does not understand the meaning of the Lord’s name, pastimes or attributes, one is purified simply by hearing or chanting of them. Such purification is called sattva-bhāvana.

One’s main purpose in human life should be to purify his existence and achieve liberation. As long as one has a material body, one is understood to be impure. In such an impure, material condition, one cannot enjoy a truly blissful life, although everyone seeks it. Therefore Śrīmad-Bhāgavatam (5.5.1) says, tapo divyaṁ putrakā yena sattvaṁ śuddhyet: one must perform tapasya, austerity, to purify his existence in order to come to the spiritual platform. The tapasya of chanting and glorifying the name, fame and attributes of the Lord is a very easy purifying process by which everyone can be happy. Therefore everyone who desires the ultimate cleansing of his heart must adopt this process. Other processes, such as karma, jñāna and yoga, cannot cleanse the heart absolutely.