In the very beginning of the Srimad Bhagavatam, in its middle and at its end, we have been repeatedly advised about the efficacy of sri nama-sankirtana.

The principle of sri nama-bhajana (loving service to the holy name) is deeply set at the root of the limb of devotion called Mathura-vasa, or sri dhama-vasa - residence in the places of Sri Bhagavan’s pastimes - for these holy places are frequently visited by sadhus who are always engaged in sri nama-sankirtana. Furthermore, the principle of chanting the holy name accompanies the service of sri murti, the transcendental deity, for it is conducted with mantras (incantations) framed with Sri Bhagavan’s names.

Thus sri nama-kirtana is glorified above all, and all success is available through it.

We also have the statement from Sri Caitanya-caritamrita (Antya-lila 4.70):

bhajanera madhye srestha nava-vidha bhakti
‘krsna-prema’, ‘krsna’ dite dhare maha-sakti
Among all forms of bhajana (loving service to Sri Krsna) the nine limbs of bhakti are best, for they have the great power to bestow both Krsna-prema and Krsna.
The foremost of these nine types of bhajana, again, is nama-sankirtana. One who chants the holy names without making spiritual offences (aparadhas) is sure to gain the wealth of transcendent love.”

In the Bhakti-sandarbha (273) we find the following statement:

Even if, in Kali-yuga, there is some other form of devotion ordained as duty, that duty should be performed along with the limb of bhakti known as kirtana.

No kind of bhakti - such as residence in Mathura, association with holy saints, service to sri murti or hearing the message of Srimad-Bhagavatam - is complete without sri Krsna nama-kirtana. Only one who gives importance to sri Krsna nama-kirtana above everything else has access to the fruit it bears.

In Sri Hari Bhakti Vilasa (11.137), a verse from an ancient scripture is cited:

yena janma-sataih purvam vasudevah samarcitah
tan-mukhe hari-namani sada tisthanti bharata
The holy names of Sri Hari are always present on the lips of a person who has been properly worshipping Him - Sri Vasudeva Krsna - for hundreds of lives.
Indeed, among all spiritual practices, sankirtana is the best and foremost means of attaining the grace of the Supreme Lord Sri Krsna.

Other types of sadhana, or devotional practices, are only worthy of being called sadhana if they favourably assist the performance of sankirtana; otherwise they should be known as obstructions to actual sadhana.

Whether one is a child or an old or young man, male or female, learned or illiterate, rich or poor, beautiful or ugly, pious or sinful - regardless of the condition of life someone may be in - there is no spiritual practice for him other than sri Krsna nama-sankirtana.

Kirtana is called sankirtana when many people perform it together.

But will it be worthy of the name sankirtana if some worthless people, without any ardour for Sri Bhagavan’s service, come together to shout in unison?

It will be true hari-sankirtana when we join the sankirtana of true devotees who are serving Sri Bhagavan according to the principles established in the Vedas and allied sastras.

Contrastingly, it is not hari-sankirtana when it is conducted for the prevention of the epidemics of cholera or pox, for prosperity in trade, for gain or for respect and fame. This kirtana is maya-kirtana - chanting within the jurisdiction of the illusory potency.

As long as we do not join the perfect Krsna-kirtana conducted by genuine devotees, maya will delude us in various ways. And if we join the kirtana of the blinded self-deceivers who are not anxious for their true well-being then surely we will not attain our own true well-being. If we follow people who never receive instructions from the true Vaisnavas but just imitate them by decorating their bodies with tulasi beads and other marks, and who shout out what appears to be names of God in external sound alone, we will not find sri hari-nama-sankirtana.