"When a person loudly chants the glories of the Lord's activities, qualities, form, etc., his chanting is called sankirtana. Sankirtana also refers to the congregational chanting of the holy name of the Lord.
In the Visnu-dharma there is a statement glorifying this process of congregational chanting: "My dear King, this word Krsna is so auspicious that anyone who chants this holy name immediately gets rid of the resultant actions of sinful activities from many, many births." That is a fact. There is the following statement in Caitanya-caritamrta: "A person who chants the holy name of Krsna once can counteract the resultant actions of more sinful activities than he is able to perform." A sinful man can perform many, many sinful activities, but he is unable to perform so many that they cannot be wiped out by one single uttering of Krsna.
In the Seventh Canto, Ninth Chapter, verse 18, of Srimad-Bhagavatam, Maharaja Prahlada offers the following prayers to the Lord: "My dear Lord Nrsimha, if I can be elevated to the position of Your servant, then it will be possible for me to hear about Your activities. You are the supreme friend, the supreme worshipable Deity. Your pastimes are transcendental, and simply by hearing of them one can counteract all his sinful activities. Therefore, I shall not care for all those sinful activities, because simply by hearing about Your pastimes I shall get out of all the contamination of material attachment."
There are many songs about the Lord's activities. For example, there is the Brahma-samhita, sung by Lord Brahma; Narada-pancaratra, sung by Narada Muni; and Srimad-Bhagavatam, sung by Sukadeva Gosvami. If these songs are heard by any person, he can easily get out of the clutches of material contamination. There should be no difficulty in hearing these songs of God. They are coming down from many, many millions of years ago, and people are still taking advantage of them. So why, at this time, should one not take full advantage and thus become liberated?
In the First Canto, Fifth Chapter, verse 22, of Srimad-Bhagavatam, Narada Muni tells his disciple Vyasadeva, "My dear Vyasa, you should know that persons who are engaged in executing austerities and penances, studying the Vedas, performing big sacrifices, chanting the hymns of the Vedas, speculating on transcendental knowledge and performing charitable functions have for all their auspicious activities simply to gain a place in the association of devotees and chant the glories of the Lord." It is indicated here that chanting about and glorifying the Lord is the ultimate activity of the living entity."
Nectar of Devotion, Chapter 9
"The transcendental vibration of hari-nama-sankirtana is imported from the spiritual world. Thus although materialists who are addicted to experimental knowledge and the so-called "scientific method" cannot place their faith in the chanting of the Hare Krsna maha-mantra, it is a fact that simply by chanting the Hare Krsna mantra offenselessly one can be freed from all subtle and gross material conditions. The spiritual world is called Vaikuntha, which means "without anxiety." In the material world everything is full of anxiety (kuntha), whereas in the spiritual world (Vaikuntha) everything is free from anxiety. Therefore those who are afflicted by a combination of anxieties cannot understand the Hare Krsna mantra, which is free from all anxiety. In the present age the vibration of the Hare Krsna maha-mantra is the only process that is in a transcendental position, beyond material contamination. Since the holy name can deliver a conditioned soul, it is explained here to be sarva-mantra-sara, the essence of all Vedic hymns."
Caitanya-caritamrta, Adi lila 7:74
Bhaktivedanta Book Trust. HDG A.C. Bhaktivedanta Swami Srila Prabhupada.