SB 6.1.15

kecit kevalayā bhaktyā
 vāsudeva-parāyaṇāḥ
aghaṁ dhunvanti kārtsnyena
 nīhāram iva bhāskaraḥ
Word for word: 
kecit — some people; kevalayā bhaktyā — by executing unalloyed devotional service; vāsudeva — to Lord Kṛṣṇa, the all-pervading Supreme Personality of Godhead; parāyaṇāḥ — completely attached (only to such service, without dependence on austerity, penance, cultivation of knowledge or pious activities); agham — all kinds of sinful reactions; dhunvanti — destroy; kārtsnyena — completely (with no possibility that sinful desires will revive); nīhāram — fog; iva — like; bhāskaraḥ — the sun.
Translation: 
Only a rare person who has adopted complete, unalloyed devotional service to Kṛṣṇa can uproot the weeds of sinful actions with no possibility that they will revive. He can do this simply by discharging devotional service, just as the sun can immediately dissipate fog by its rays.
Purport: 

In the previous verse Śukadeva Gosvāmī gave the example that the dried leaves of creepers beneath a bamboo tree may be completely burnt to ashes by a fire, although the creepers may sprout again because the root is still in the ground. Similarly, because the root of sinful desire is not destroyed in the heart of a person who is cultivating knowledge but who has no taste for devotional service, there is a possibility that his sinful desires will reappear. As stated in Śrīmad-Bhāgavatam (10.14.4):

śreyaḥ-sṛtiṁ bhaktim udasya te vibho
 kliśyanti ye kevala-bodha-labdhaye

Speculators who undergo great labor to gain a meticulous understanding of the material world by distinguishing between sinful and pious activities, but who are not situated in devotional service, are prone to material activities. They may fall down and become implicated in fruitive activities. If one becomes attached to devotional service, however, his desires for material enjoyment are automatically vanquished without separate endeavor. Bhaktiḥ pareśānubhavo viraktir anyatra ca: if one is advanced in Kṛṣṇa consciousness, material activities, both sinful and pious, automatically become distasteful to him. That is the test of Kṛṣṇa consciousness. Both pious and impious activities are actually due to ignorance because a living entity, as an eternal servant of Kṛṣṇa, has no need to act for his personal sense gratification. Therefore as soon as one is reclaimed to the platform of devotional service, he relinquishes his attachment for pious and impious activities and is interested only in what will satisfy Kṛṣṇa. This process of bhakti, devotional service to Kṛṣṇa (vāsudeva-parāyaṇa), relieves one from the reactions of all activities.

Since Mahārāja Parīkṣit was a great devotee. the answers of his guru, Śukadeva Gosvāmī, concerning karma-kāṇḍa and jñāna-kāṇḍa could not satisfy him. Therefore Śukadeva Gosvāmī, knowing very well the heart of his disciple, explained the transcendental bliss of devotional service. The word kecit, which is used in this verse, means. “a few people but not all.” Not everyone can become Kṛṣṇa conscious. As Kṛṣṇa explains in Bhagavad-gītā (7.3):

manuṣyāṇāṁ sahasreṣu
 kaścid yatati siddhaye
yatatām api siddhānāṁ
 kaścin māṁ vetti tattvataḥ

“Out of many thousands among men, one may endeavor for perfection, and of those who have achieved perfection, hardly one knows Me in truth.” Practically no one understands Kṛṣṇa as He is, for Kṛṣṇa cannot be understood through pious activities or attainment of the most elevated speculative knowledge. Actually the highest knowledge consists of understanding Kṛṣṇa. Unintelligent men who do not understand Kṛṣṇa are grossly puffed up, thinking that they are liberated or have themselves become Kṛṣṇa or Nārāyaṇa. This is ignorance.

To indicate the purity of bhakti, devotional service, Śrīla Rūpa Gosvāmī says in Bhakti-rasāmṛta-sindhu (1.1.11):

anyābhilāṣitā-śūnyaṁ
 jñāna-karmādy-anāvṛtam
ānukūlyena kṛṣṇānu-
 śīlanaṁ bhaktir uttamā

“One should render transcendental loving service to the Supreme Lord Kṛṣṇa favorably and without desire for material profit or gain through fruitive activities or philosophical speculation. That is called pure devotional service.” Śrīla Rūpa Gosvāmī further explains that bhakti is kleśaghnī śubhadā, which means if one takes to devotional service, all kinds of unnecessary labor and material distress cease entirely and one achieves all good fortune. Bhakti is so powerful that it is also said to be mokṣa-laghutākṛt; in other words, it minimizes the importance of liberation.

Nondevotees must undergo material hardships because they are prone to commit sinful fruitive activities. The desire to commit sinful actions continues in their hearts due to ignorance. These sinful actions are divided into three categories — pātaka, mahā-pātaka and atipātaka — and also into two divisions; prārabdha and aprārabdha. Prārabdha refers to sinful reactions from which one is suffering at the present, and aprārabdha refers to sources of potential suffering. When the seeds (bīja) of sinful reactions have not yet fructified, the reactions are called aprārabdha. These seeds of sinful action are unseen, but they are unlimited, and no one can trace when they were first planted. Because of prārabdha, sinful reactions that have already fructified, one is seen to have taken birth in a low family or to be suffering from other miseries.

When one takes to devotional service, however, all phases of sinful life, including prārabdha, aprārabdha and bīja, are vanquished. In Śrīmad-Bhāgavatam (11.14.19) Lord Kṛṣṇa tells Uddhava:

yathāgniḥ susamṛddhārciḥ
 karoty edhāṁsi bhasmasāt
tathā mad-viṣayā bhaktir
 uddhavaināṁsi kṛtsnaśaḥ

“My dear Uddhava, devotional service in relationship with Me is like a blazing fire that can burn to ashes all the fuel of sinful activities supplied to it.” How devotional service vanquishes the reactions of sinful life is explained in Śrīmad-Bhāgavatam (3.33.6) in a verse spoken during Lord Kapiladeva’s instructions to His mother, Devahūti. Devahūti said:

yan-nāmadheya-śravaṇānukīrtanād
 yat-prahvaṇād yat-smaraṇād api kvacit
śvādo ’pi sadyaḥ savanāya kalpate
 kutaḥ punas te bhagavan nu darśanāt

“My dear Lord, if even a person born in a family of dog-eaters hears and repeats the chanting of Your glories, offers respects to You and remembers You, he is immediately greater than a brāhmaṇa and is therefore eligible to perform sacrifices. Therefore, what is to be said of one who has seen You directly?”

In the Padma Purāṇa there is a statement that persons whose hearts are always attached to the devotional service of Lord Viṣṇu are immediately released from all the reactions of sinful life. These reactions generally exist in four phases. Some of them are ready to produce results immediately, some are in the form of seeds, some are unmanifested, and some are current. All such reactions are immediately nullified by devotional service. When devotional service is present in one’s heart, desires to perform sinful activities have no place there. Sinful life is due to ignorance, which means forgetfulness of one’s constitutional position as an eternal servant of God, but when one is fully Kṛṣṇa conscious he realizes that he is God’s eternal servant.

In this regard, Śrīla Jīva Gosvāmī comments that bhakti may be divided into two divisions: (1) santatā, devotional service that continues incessantly with faith and love, and (2) kādācitkī, devotional service that does not continue incessantly but is sometimes awakened. Incessantly flowing devotional service (santatā) may also be divided into two categories: (1) service performed with slight attachment and (2) spontaneous devotional service. Intermittent devotional service (kādācitkī) may be divided into three categories: (1) rāgābhāsamayī, devotional service in which one is almost attached, (2) rāgābhāsa-śūnya-svarūpa-bhūtā, devotional service in which there is no spontaneous love but one likes the constitutional position of serving, and (3) ābhāsa-rūpā, a slight glimpse of devotional service. As for atonement, if one has caught even a slight glimpse of devotional service, all needs to undergo prāyaścitta, atonement, are superseded. Therefore atonement is certainly unnecessary when one has achieved spontaneous love and, above that, attachment with love, which are signs of increasing advancement in kādācitkī. Even in the stage of ābhāsa-rūpā bhakti, all the reactions of sinful life are uprooted and vanquished. Śrīla Jīva Gosvāmī expresses the opinion that the word kārtsnyena means that even if one has a desire to commit sinful actions, the roots of that desire are vanquished merely by ābhāsa-rūpā bhakti. The example of bhāskara, the sun, is most appropriate. The ābhāsa feature of bhakti is compared to dawn, and the accumulation of one’s sinful activities is compared to fog. Since fog does not spread throughout the sky, the sun need do no more than merely manifest its first rays, and the fog immediately disappears. Similarly, if one has even a slight relationship with devotional service, all the fog of his sinful life is immediately vanquished.