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
Srimad Bhagavatam it is repeatedly and clearly said that the essence of religion in the Age of Kali is the chanting of the holy name of Krishna.
Caitanya Caritamrta Adi lila 3.50
In its present condition, the world can only be saved by introducing this sankirtana, this Krishna consciousness movement.
Purport on Srimad Bhagavatam 4.18.8
When a Vedic mantra is chanted within the mouth so that only the chanter can hear, the chanting is called japa. But when such mantras are chanted loudly, it is called kirtana.
Purport on Srimad Bhagavatam 3.14.32
Lord Chaitanya, however, has recommended only one yajna, or sacrifice, called the sankirtana-yajna, the chanting of Hare Krishna, in which everyone can take part. Thus both devotees and fruitive workers can derive equal benefit from the performances of sankirtana-yajna.
Purport on Srimad bhagavatam 1.16.20
Love of God is transcendental to liberation [mukti], and thus it is called the fifth stage of spiritual realization, above the stage of liberation.
Srimad Bhagavatam Introduction.
In this age of Kali there is no other religion but the glorification of the Lord by utterance of His holy name, and that is the injunction of all the revealed scriptures.