CC Madhya 23.100
sarvathaiva durūho ’yam
sarvathā — in all respects; eva — certainly; durūhaḥ — difficult to be understood; ayam — this; abhaktaiḥ — by nondevotees; bhagavat-rasaḥ — the exchange of transcendental mellows with the Supreme Personality of Godhead; tat — that; pāda-ambuja-sarva-svaiḥ — whose all in all is the lotus feet; bhaktaiḥ — by devotees; eva — certainly; anurasyate — is relished.
“ ‘Nondevotees cannot understand the exchange of transcendental mellows between the devotees and the Lord. In all respects, this is very difficult to understand, but one who has dedicated everything to the lotus feet of Kṛṣṇa can taste the transcendental mellows.’
This verse is also found in the Bhakti-rasāmṛta-sindhu (2.5.131).