Sri Padma Purana states: satam ninda namnah paramam aparadham vitanute / yatah khyatim yatam katham u sahate tad-vigarham. To blaspheme the great saintly persons who are engaged in preaching the glories of the Hare Krsna maha-mantra is the worst offense at the lotus feet of the holy name. One should not criticize a preacher of the glories of the Hare Krsna maha-mantra. If one does so, he is an offender. The Nama-prabhu, who is identical with Krsna, will never tolerate such blasphemous activities, even from one who passes as a great devotee.

Srila Prabhupada – CC Adi 8.24 purport.