If one chants the holy name of the Lord just to make a show, not knowing the secret of success, he may increase his bile secretion, but he will never attain perfection in chanting the holy name.
Purport on Chaitanya Caritamrta Adi Lila 7.95-96
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
By performing congregational chanting of the Hare Krishna mantra, one can destroy the sinful condition of material existence, purify the unclean heart and awaken all varieties of devotional service.
Chaitanya Caritamrta on Antya Lila 20.13
Of the nine processes of devotional service, the most important is to always chant the holy name of the Lord. If one does so, avoiding the ten kinds of offenses, one very easily obtained the most valuable love of Godhead.
Chaitanya Caritamrta Antya Lila 4.71