Whenever we find someone extraordinary preaching the bhakti cult, we should know that he is especially empowered by Lord Visnu, or Lord Krsna. As confirmed in Caitanya-caritamrta (Antya 7.11), krsna-sakti vina nahe tara pravartana: one cannot explain the glories of the holy name of the Lord without being specifically empowered by Him. If one criticizes or finds fault with such an empowered personality, one is to be considered an offender against Lord Visnu and is punishable. Even though such offenders may dress as Vaisnavas with false tilaka and mala, they are never forgiven by the Lord if they offend a pure Vaisnava. There are many instances of this in the sastras.

From Srila Prabhupada’s purport to SB 4.19.37