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.
Just as fire applied to gold removes any discoloration caused by traces of other metals, Lord Vishnu within the heart purifies the minds of the yogis.