kṛṣṇera - of Lord Kṛṣṇa; pratijñā - promise; eka - one; āche - there is; pūrva haite - from before; ye - whoever; yaiche - just as; bhaje - he worships; kṛṣṇa - Lord Kṛṣṇa; tāre - to him; bhaje - reciprocates; taiche - just so.

Lord Kṛṣṇa has a promise from before to reciprocate with His devotees according to the way they worship Him.