CC Ādi 3.40

kali-yuge yuga-dharma — nāmera pracāra
tathi lāgi’ pīta-varṇa caitanyāvatāra
Word for word: 
kali-yuge — in the Age of Kali; yuga-dharma — the religious practice for the age; nāmera — of the holy name; pracāra — propagation; tathi — this; lāgi’ — for; pīta-varṇa — having a yellow color; caitanya-avatāra — the incarnation of Lord Caitanya.
Translation: 
The religious practice for the Age of Kali is to broadcast the glories of the holy name. Only for this purpose has the Lord, in a yellow color, descended as Lord Caitanya.
Purport: 

In this Age of Kali the practical system of religion for everyone is the chanting of the name of Godhead. This was introduced in this age by Lord Caitanya. Bhakti-yoga actually begins with the chanting of the holy name, as confirmed by Madhvācārya in his commentary on the Muṇḍaka Upaniṣad. He quotes this verse from the Nārāyaṇa-saṁhitā:

dvāparīyair janair viṣṇuḥpañcarātrais tu kevalaiḥ
kalau tu nāma-mātreṇa
pūjyate bhagavān hariḥ

“In the Dvāpara-yuga people should worship Lord Viṣṇu only by the regulative principles of the Nārada-pañcarātra and other such authorized books. In the Age of Kali, however, people should simply chant the holy names of the Supreme Personality of Godhead.” The Hare Kṛṣṇa mantra is specifically mentioned in many Upaniṣads, such as the Kali-santaraṇa Upaniṣad, where it is said:

hare kṛṣṇa hare kṛṣṇakṛṣṇa kṛṣṇa hare hare
hare rāma hare rāma
rāma rāma hare hare

iti ṣoḍaśakaṁ nāmnāṁkali-kalmaṣa-nāśanaṁ
nātaḥ parataropāyaḥ
sarva-vedeṣu dṛśyate

“After searching through all the Vedic literature, one cannot find a method of religion more sublime for this age than the chanting of Hare Kṛṣṇa.”