SB 6.14.1

śrī-parīkṣid uvāca
rajas-tamaḥ-svabhāvasya
 brahman vṛtrasya pāpmanaḥ
nārāyaṇe bhagavati
 katham āsīd dṛḍhā matiḥ
Word for word: 
śrī-parīkṣit uvāca — King Parīkṣit inquired; rajaḥ — of the mode of passion; tamaḥ — and of the mode of ignorance; sva-bhāvasya — having a nature; brahman — O learned brāhmaṇa; vṛtrasya — of Vṛtrāsura; pāpmanaḥ — who was supposedly sinful; nārāyaṇe — in Lord Nārāyaṇa; bhagavati — the Supreme Personality of Godhead; katham — how; āsīt — was there; dṛḍhā — very strong; matiḥ — consciousness.
Translation: 
King Parīkṣit inquired from Śukadeva Gosvāmī: O learned brāhmaṇa, demons are generally sinful, being obsessed with the modes of passion and ignorance. How, then, could Vṛtrāsura have attained such exalted love for the Supreme Personality of Godhead, Nārāyaṇa?
Purport: 

In this material world, everyone is obsessed with the modes of passion and ignorance. However, unless one conquers these modes and comes to the platform of goodness, there is no chance of one’s becoming a pure devotee. This is confirmed by Lord Kṛṣṇa Himself in Bhagavad-gītā (7.28):

yeṣāṁ tv anta-gataṁ pāpaṁ
 janānāṁ puṇya-karmaṇām
te dvandva-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 delusion, engage themselves in My service with determination.” Since Vṛtrāsura was among the demons, Mahārāja Parīkṣit wondered how it was possible for him to have become such an exalted devotee.