Spiritual enlightenment is possible by the mercy of the spiritual energy of the Lord. The chanting of the Hare Kṛṣṇa mantra is first addressed to the spiritual energy of the Lord, Hare. This spiritual energy acts when a living entity fully surrenders and accepts his position as an eternal servitor. When a person places himself at the disposal or order of the Supreme Lord, that is called sevonmukha; at that time the spiritual energy gradually reveals the Lord to him.
(Srimad Bhagavatam---4:9:6---purport).

In the Bhagavad-gītā the Lord says that to those who are constantly engaged in devotional service with love and affection, the Supreme Personality of Godhead gives intelligence from within, and thus they may make further progress. Being so encouraged, the devotee can never forget, at any moment, the Personality of Godhead. He always feels obliged to Him for having achieved increased power in devotional service by His grace. Saintly persons like Sanaka, Sanātana and Lord Brahmā were able to see the entire universe, by the mercy of the Lord, through knowledge of the Lord. The example is given that a person may apparently abstain from sleep all day, but as long as he is not spiritually enlightened he is actually sleeping. He may sleep at night and perform his duties in the daytime, but as long as he does not come to the platform of working in spiritual enlightenment he is considered to be always sleeping. A devotee, therefore, never forgets the benefit derived from the Lord.
(Srimad Bhagavatam---4:9:8---purport).

Because a person who is spiritually advanced accepts any adverse condition of life as the mercy of the Lord, he is completely eligible to enter into the spiritual kingdom. Even though a person takes to the devotional service of the Supreme Lord, he may sometimes become diseased, impoverished, or disappointed by life's events. A true devotee of the Lord always considers these sufferings to be due to past sinful activities, and thus without becoming disturbed he patiently awaits the mercy of the Supreme Lord. Such devotees are compared to high mountains, which are never agitated in any way, even when struck by powerful torrents of rain in the rainy season. Rather, such devotees remain humble inspiritual enlightenment. Free from pride and envy, they easily gain the mercy of the Lord and go back home, back to Godhead.
(Light of the Bhagavata).

In Bhagavad-gītā (10.11) it is said:
teṣām evānukampārtham
aham ajñāna-jaṁ tamaḥ
nāśayāmy ātma-bhāva stho
jñāna-dīpena bhāsvatā
For a devotee who has taken the lotus feet of the Lord within his heart, the Lord gives spiritual enlightenment, known as jñāna-dīpa, by special mercy from within. This jñāna-dīpa is compared to the fire hidden within araṇi wood. To perform fire sacrifices, great sages previously did not ignite a fire directly; the fire would be invoked from araṇi wood. Similarly, all living entities are covered by the modes of material nature, and the fire of knowledge can be ignited only by the Supreme Personality of Godhead when one takes Him within one's heart.
(Srimad Bhagavatam---8:3:16---purport).