SB 5.1.5

śrī-śuka uvāca
bāḍham uktaṁ bhagavata uttamaślokasya śrīmac-caraṇāravinda-makaranda-rasa āveśita-cetaso bhāgavata-paramahaṁsa-dayita-kathāṁ kiñcid antarāya-vihatāṁ svāṁ śivatamāṁ padavīṁ na prāyeṇa hinvanti.
Word for word: 
śrī-śukaḥ uvāca — Śrī Śukadeva Gosvāmī said; bāḍham — correct; uktam — what you have said; bhagavataḥ — of the Personality of Godhead; uttama-ślokasya — who is praised with excellent verses; śrīmat-caraṇa-aravinda — of the feet, which are just like the most beautiful fragrant lotus flowers; makaranda — honey; rase — in the nectar; āveśita — absorbed; cetasaḥ — whose hearts; bhāgavata — to the devotees; paramahaṁsa — liberated persons; dayita — pleasing; kathām — glorification; kiñcit — sometimes; antarāya — by impediments; vihatām — checked; svām — own; śiva-tamām — most exalted; padavīm — position; na — do not; prāyeṇa — almost always; hinvanti — give up.
Translation: 
Śrī Śukadeva Gosvāmī said: What you have said is correct. The glories of the Supreme Personality of Godhead, who is praised in eloquent, transcendental verses by such exalted personalities as Brahmā, are very pleasing to great devotees and liberated persons. One who is attached to the nectarean honey of the Lord’s lotus feet, and whose mind is always absorbed in His glories, may sometimes be checked by some impediment, but he still never gives up the exalted position he has acquired.
Purport: 

Śrī Śukadeva Gosvāmī accepted both of the King’s propositions — that a person who is advanced in Kṛṣṇa consciousness cannot embrace materialistic life again and that one who has embraced materialistic life cannot take up Kṛṣṇa consciousness at any stage of his existence. Although accepting both these statements, Śukadeva Gosvāmī qualified them by saying that a person who has once absorbed his mind in the glories of the Supreme Personality of Godhead may sometimes be influenced by impediments but he still does not give up his exalted devotional position.

According to Śrīla Viśvanātha Cakravartī Ṭhākura, there are two kinds of impediments to devotional service. The first is an offense at the lotus feet of a Vaiṣṇava. This is called vaiṣṇava-aparādha. Śrī Caitanya Mahāprabhu warned His devotees not to commit vaiṣṇava-aparādha, which He described as the mad elephant offense. When a mad elephant enters a beautiful garden, it destroys everything, leaving a barren field. Similarly, the power of vaiṣṇava-aparādha is so great that even an advanced devotee becomes almost devoid of his spiritual assets if he commits it. Since Kṛṣṇa consciousness is eternal, it cannot be destroyed altogether, but advancement may be checked for the time being. Thus vaiṣṇava-aparādha is one kind of impediment to devotional service. Sometimes, however, the Supreme Personality of Godhead or His devotee desires to impede one’s devotional service. For example, Hiraṇyakaśipu and Hiraṇyākṣa were formerly Jaya and Vijaya, the gatekeepers in Vaikuṇṭha, but by the desire of the Lord they became His enemies for three lives. Thus the desire of the Lord is another kind of impediment. But in both cases, the pure devotee, once advanced in Kṛṣṇa consciousness, cannot be lost. Following the orders of his superiors (Svāyambhuva and Lord Brahmā), Priyavrata accepted family life, but this did not mean he lost his position in devotional service. Kṛṣṇa consciousness is perfect and eternal, and therefore it cannot be lost under any circumstances. Because the material world is full of obstructions to advancement in Kṛṣṇa consciousness, there may appear to be many impediments, yet Kṛṣṇa, the Supreme Personality of Godhead, declares in Bhagavad-gītā (9.31), kaunteya pratijānīhi na me bhaktaḥ praṇaśyati: once one has taken shelter at the lotus feet of the Lord, he cannot be lost.

In this verse, the word śivatamām is very significant. Śivatamām means “the most auspicious.” The devotional path is so auspicious that a devotee cannot be lost under any circumstances. This is described in the Śrīmad Bhagavad-gītā by the Lord Himself. Pārtha naiveha nāmutra vināśas tasya vidyate: “My dear Arjuna, for a devotee there is no question of being lost, either in this life or in the next.” (Bg. 6.40) In Bhagavad-gītā (6.43) the Lord clearly explains how this is so.

tatra taṁ buddhi-saṁyogaṁ
 labhate paurva-dehikam
yatate ca tato bhūyaḥ
 saṁsiddhau kuru-nandana

By the order of the Lord, a perfect devotee sometimes comes to this material world like an ordinary human being. Because of his previous practice, such a perfect devotee naturally becomes attached to devotional service, apparently without cause. Despite all kinds of impediments due to surrounding circumstances, he automatically perseveres in devotional service and gradually advances until he once again becomes perfect. Bilvamaṅgala Ṭhākura had been an advanced devotee in his previous life, but in his next life he became greatly fallen and was attached to a prostitute. Suddenly, however, his entire behavior was changed by the words of the very prostitute who had so much attracted him, and he became a great devotee. In the lives of exalted devotees, there are many such instances, proving that once one has taken to the shelter of the lotus feet of the Lord he cannot be lost (kaunteya pratijānīhi na me bhaktaḥ praṇaśyati).

The fact is, however, that one becomes a devotee when he is completely freed from all reactions to sinful life. As Kṛṣṇa states in Bhagavad-gītā (7.28):

yeṣāṁ tv anta-gataṁ pāpaṁ
 janānāṁ puṇya-karmaṇām
te dvanda-moha-nirmuktā
 bhajante māṁ dṛḍha-vratāḥ

“Persons who have acted piously in previous lives and in this life, whose sinful actions are completely eradicated and who are freed from the duality of illusion engage themselves in My service with determination.” On the other hand, as Prahlāda Mahārāja said:

matir na kṛṣṇe parataḥ svato vā
 mitho ’bhipadyeta gṛha-vratānām

A person who is too attached to materialistic family life — home, family, wife, children and so on — cannot develop Kṛṣṇa consciousness.

These apparent contradictions are resolved in the life of a devotee by the grace of the Supreme Lord, and therefore a devotee is never bereft of his position on the path of liberation, which is described in this verse as śivatamāṁ padavīm.