Unless one receives the mercy of the Lord, he cannot understand the Supreme Personality of Godhead or engage in His devotional service.

Only a devotee who has received a small fraction of the mercy of the Lord can understand Him. Others may engage in theoretical speculation to understand the Lord, but they can not know anything about Him. Although Maharaja Prataparudra was very eager to see Sri Caitanya Mahaprabhu, the Lord refused to see him. However, when Sri Caitanya Mahaprabhu saw the King engaged in menial service for Lord Jagannatha, He became very happy. Thus the King became eligible to receive Sri Caitanya Mahaprabhu's mercy. If a devotee accepts Lord Sri Caitanya Mahaprabhu as the universal guru and Lord Jagannatha as the Supreme Personality of Godhead Krsna, he is benefited by the combined mercy of Krsna and guru. That is stated by Sri Caitanya Mahaprabhu in His instructions to Rupa Gosvami. (Cc. Madhya 19.151)

The seed of devotional service fructifies and becomes a transcendental creeper. Finally it reaches the lotus feet of the Lord in the spiritual sky. This seed is obtained by the mercy of the Lord and the guru. By the Lord's mercy one gets the association of a bona fide guru, and by the mercy of the guru, one gets a chance to render devotional service. Devotional service, the science of bhakti-yoga, carries one from this material world to the spiritual world.