what a nice explanation from Srila Ananta das Babaji:
"1) SADHU NINDA (BLASPHEMING THE SAINT): Blaspheming the saint is a grave offense, and is called mahad aparddha. A sddhu is a devotee of God through whose preaching the holy name has become world famous. How can the holy name tolerate blasphemy of such a person? Many people think that there is no harm in discussing the fault of some devotee who is engaged in some wicked activities, because that criticism is true and justified, but Sripada Sridhara SvamI writes in his commentary on SrimadBhdgavata: nindanam dosa kirtanam - "There is no question whether the devotee who is under discussion is at fault or not, speaking bad about such a person is called blasphemy." It is natural that we wonder: "Who are these devotees, offending whom we commit an offense to the holy name?" In his book Mddhurya Kddambini, Srila Visvanatha CakravartI gives the answer: "It is not proper to think that only those devotees who are merciful, nonviolent, tolerant etc. are to be considered saints, and that those who are not endowed with such qualities are not saints, and that one can thus blaspheme them without committing an offense to the holy name. Actually, anyone who worships God, be he deceitful, ill-behaved, hypocritical and unclean, is a saint, and to blaspheme or criticise such a person is an offense to the holy name. Sri Krsna Himself says in Hhagavad Gitd (9.30):
apt cet suduracdro bhajate mam ananya bhdk sddhur eva sa mantavyah samyag vyavasito hi sah
"Even if one commits the most abominable activities, he is to be considered a saint when he exclusively worships Me, for he is on the right path." Those who have fixed the thought in their mind, "I will be blessed by worshipping the Supreme Lord", and who do not worship demigods, but only the Supreme Lord, are saints, even if they misbehave, and to criticise even them is an offense to the holy name."