CB Madhya 13.312

nindāya nā bāḍe dharma—sabe pāpa lābha
eteke nā kare nindā saba mahābhāga
Blasphemy does not enhance one’s religious principles, it only results in sin. That is why fortunate souls do not engage in blasphemy.

Purport by Srila Bhaktisiddhanta Saraswati Thakura:

The devotees of the Lord do not blaspheme anyone in this world. One who blasphemes others is known as “sinful” or “irreligious.”

To attribute on someone’s faults that are not present is called blasphemy.

Being induced by irrelevant goals, those who are unable to tolerate the glorification of others and with a desire to attack them unfairly attribute faults on them continually meet with inauspiciousness day after day.

A person who out of envy attributes faults on an irreproachable Vaiṣṇava must go to the hell known as Kumbhīpāka and suffer severe miseries.

Those sinful persons who cannot understand that sarva-mahāguṇa-gaṇa vaiṣṇava-śarīre—“all good qualities are present in the body of a Vaiṣṇava,” and who consider that non-Vaiṣṇavas are equal to Vaiṣṇavas can never achieve any benefit.

To criticize the behavior of nondevotees is called sad-upadeśa, or good instruction.

Other than devotional service to Viṣṇu, all activities of the living entities are condemnable. Sinful people often engage in abominable activities on the pretext of devotional service to Viṣṇu. Instructions to give up these activities should not be called nindā, or blasphemy.