CC Madhya 19.166
ataeva śuddha-bhaktira kahiye ‘lakṣaṇa’
“When one is situated in pure devotional service, he develops love of Godhead; therefore let me describe some of the symptoms of pure devotional service.
In the Bhagavad-gītā (18.55) it is said, bhaktyā mām abhijānāti yāvān yaś cāsmi tattvataḥ. One cannot understand the Supreme Personality of Godhead in truth unless he takes to devotional service.