There are two kinds of highly elevated devotees, called sadhana-siddha and kripa-siddha. Sadhana-siddha refers to one who has become a devotee by regular execution of the regulative principles mentioned in the sastras, as ordered and directed by the spiritual master. If one regularly executes such devotional service, he will certainly attain perfection in due course of time. But there are other devotees, who may not have undergone all the required details of devotional service but who, by the special mercy of guru and Krishna–the spiritual master and the Supreme Personality of Godhead–have immediately attained the perfection of pure devotional service. Examples of such devotees are the yajna-patnis, Maharaja Bali and Sukadeva Gosvami. The yajna-patnis were the wives of ordinary brahmanas engaged in fruitive activities. Although the brahmanas were very learned and advanced in Vedic knowledge, they could not achieve the mercy of Krishna-Balarama, whereas their wives achieved complete perfection in devotional service, despite their being women. Similarly, Vairocani, Bali Maharaja, received the mercy of Prahlada Maharaja, and by Prahlada Maharaja’s mercy he also received the mercy of Lord Vishnu, who appeared before him as a brahmacari beggar. Thus Bali Maharaja became a kripa-siddha because of the special mercy of both guru and Krishna. Caitanya Mahaprabhu confirms this favor: guru-Krishna-prasade paya bhakti-lata-bija (Cc. Madhya 19.151). Bali Maharaja, by the grace of Prahlada Maharaja, got the seed of devotional service, and when that seed developed, he achieved the ultimate fruit of that service, namely love of Godhead (prema pum-artho mahan), immediately upon the appearance of Lord Vamanadeva. Bali Maharaja regularly maintained devotion for the Lord, and because he was purified, the Lord appeared before him. Because of unalloyed love for the Lord, he then immediately decided, “I shall give this little dwarf brahmana whatever He asks from me.” This is a sign of love. Thus Bali Maharaja is understood to be one who received the highest perfection of devotional service by special mercy.
Source: A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “Srimad Bhagavatam”, Eighth Canto, Chapter 20 – Text 3