Caitanya Caritamrta on Progressing in Spiritual Life in Different Yugas: For progress in spiritual life, the sastras recommend meditation in Satya-yuga, sacrifice for the satisfaction of Lord Visnu in Treta-yuga and gorgeous worship of the Lord in the temple in Dvapara-yuga, but in the Age of Kali one can achieve spiritual progress only by chanting the holy name of the Lord.
Purport on Caitanya Caritamrta Adi Lila 7.76
Prithu Maharaja is very emphatic in this connection — only an animal can give up the practice of chanting Hare Krishna. Those who are not animals but actually intelligent, advanced, human, civilized men cannot give up this practice of continually chanting Hare Krishna, Hare Krishna, Krishna Krishna, Hare Hare/ Hare Rama, Hare Rama, Rama Rama, Hare Hare.
Purport on Srimad Bhagavatam 4.20.26
In this horrible condition of life there is only one way of liberation—to engage in the activities of transcendental chanting and hearing of the holy name of the Supreme Lord.
Purport on Srimad Bhagavatam 4.7.44
The main purpose of sannyasa life is to be in constant companionship with the Supreme Lord, either by thinking of Him within the heart or hearing of Him through aural reception.
Purport on SB 3.24.35