CC Madhya 20.339

ei mantre dvāpare kare kṛṣṇārcana
‘kṛṣṇa-nāma-saṅkīrtana’ — kali-yugera dharma
Word for word: 
ei mantre — by this mantra; dvāpare — in the Age of Dvāpara; kare — perform; kṛṣṇa-arcana — the worship of Lord Kṛṣṇa; kṛṣṇa-nāma-saṅkīrtana — chanting of the holy name of Lord Kṛṣṇa; kali-yugera dharma — the occupational duty in the Age of Kali.
Translation: 
“By this mantra, the people worship Lord Kṛṣṇa in Dvāpara-yuga. In Kali-yuga the occupational duty of the people is to chant congregationally the holy name of Kṛṣṇa.
Purport: 

As stated in Śrīmad-Bhāgavatam (12.3.51):

kaler doṣa-nidhe rājannasti hy eko mahān guṇaḥ
kīrtanād eva kṛṣṇasya
mukta-bandhaḥ paraṁ vrajet

“My dear King, although Kali-yuga is full of faults, there is still one good quality about this age. It is that simply by chanting the Hare Kṛṣṇa mahā-mantra, one can become free from material bondage and be promoted to the transcendental kingdom.” Thus in Kali-yuga one worships Lord Kṛṣṇa by chanting Hare Kṛṣṇa, Hare Kṛṣṇa, Kṛṣṇa Kṛṣṇa, Hare Hare/ Hare Rāma, Hare Rāma, Rāma Rāma, Hare Hare. To propagate this movement, Lord Kṛṣṇa personally appeared as Lord Caitanya Mahāprabhu. That is described in the following verse.