CC Madhya 20.339
‘kṛṣṇa-nāma-saṅkīrtana’ — kali-yugera dharma
“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.
As stated in Śrīmad-Bhāgavatam (12.3.51):
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.