Faithfulness
When we face difficulties and still remain faithful to Krishna and the spiritual master, we access the infinite mercy of their hearts.
Significance of The Mercy of A Pure Devotee
Krishna does not agree to award devotional service to merely anyone. Krishna can easily offer a person material happiness or even liberation, but He does not agree very easily to award a person engagement in his devotional service. Devotional service can in fact be attained only through the mercy of a pure devotee. — The Nectar of Devotion, Page 14.
Eligibility For Going Back To Godhead
To travel in another country, you need visa permission, similarly to go back to Godhead you need permission & mercy from his pure devotee.
Causeless Mercy of Lord Visnu
The causeless mercy of Lord Vishnu is unparalleled, and such mercy is perceived by the devotees only by the grace of the Lord.
Srimad Bagavatam 1.6.31,
Srila Prabhupada.
Special Mercy
To show them special mercy, I, dwelling in their hearts, destroy with the shining lamp of knowledge the darkness born of ignorance.
Bhagavad Gita 10.11,
Srila Prabhupada.
Grace of The Spiritual Master
By the grace of the spiritual master the cloud of the mercy of the Personality of Godhead is brought in, and then only, when the rains of Krishna consciousness fall, can the fire of material existence be extinguished.
Srimad Bhagavatam 3.21.17,
Srila Prabhupada.