CC Antya 10.100

ei saba haya bhakti-śāstra-sūkṣma marma
caitanyera kṛpāya jāne ei saba dharma
Synonyms: 
ei saba — all these; haya — are; bhakti-śāstra — of the system of devotional service; sūkṣma marma — finer principles; caitanyera kṛpāya — by the mercy of Śrī Caitanya Mahāprabhu; jāne — one can understand; ei saba — all these; dharma — principles of devotional service.
Translation: 
These are some of the finer points of etiquette in devotional service. Only one who has received the mercy of Śrī Caitanya Mahāprabhu can understand these principles.
Purport: 

Karmīs, fruitive workers, cannot understand the finer conclusions of devotional service because they accept only its ritualistic value but do not understand how devotional service satisfies the Supreme Personality of Godhead. The karmīs view the formalities as a means of advancing in religion, economic development, sensual satisfaction and liberation. Although these are only material results of following religious principles, the karmīs consider them everything. Such ritualistic activities are called karma. Karmīs who adopt devotional service very loosely and who therefore remain on the platform of material activities are called prākṛta-sahajiyās. They cannot understand how pure devotional service is rendered in parental and conjugal love, for this can be understood only by the special mercy bestowed by Śrī Caitanya Mahāprabhu upon pure devotees.