13 To preach effectively, one must chant the holy name
In order to have potency to preach, the preachers of the Krsna consciousness movement should become purified by chanting the holy name:
Our preachers who are preaching Krsna consciousness all over the world should follow in the footsteps of Narada Muni and become purified by following the four principles and chanting the Hare Krsna maha−mantra. This will make them fit to become Vaisnavas. Then, when they speak to sinful people about the teachings of this Krsna consciousness movement, people will be affected and take the instructions.
– Sri Caitanya caritamrta Madhya lila 24.252
By chanting sixteen rounds of the maha−mantra (etc.) one gets spiritual strength with which to preach Krsna consciousness all over the world:
[Chand Kazi to the devotees of Lord Caitanya]: "For so long you did not follow the regulative principles of the Hindu religion, but now you are following them with great enthusiasm. May I know by whose strength you are doing so?"
It appears that from the aggression of Vaktiyara Khiliji in Bengal until the time of Chand Kazi, Hindus, or the followers of the Vedic principles, were greatly suppressed. Like the Hindus in present−day Pakistan, practically no one could execute the Hindu religious principles freely. Chand Kazi referred to this condition of Hindu society. Formerly the Hindus had not been straightforward in executing their Hindu principles, but now they were freely chanting the Hare Krsna maha−mantra. Therefore it must have been by the strength of someone else that they were so daring.
Actually, that was the fact. Although the members of the so−called Hindu society had followed the social customs and formulas, they had practically forgotten to execute their religious principles strictly. But with the presence of Sri Caitanya Mahaprabhu they actually began following the regulative principles according to His order. That order is still existing, and anywhere and everywhere, in all parts of the world, one can execute it. That order is to become a spiritual master under the direction of Sri Caitanya Mahaprabhu by following the regulative principles, chanting daily at least sixteen rounds of the Hare Krsna maha−mantra, and preaching the cult of Krsna consciousness all over the world. If we adhere to the order of Sri Caitanya Mahaprabhu, we shall get spiritual strength without a doubt, and we shall be free to preach this cult of the Hare Krsna movement and not be hampered by anyone.
– Sri Caitanya caritamrta Adi lila 17.126