As Muslims, the default assumption we should have about other people in any matter is that they are free of blame. Islam demands fairness and impartiality when it comes to judging others.
Allah says: "And when you speak, then be just, though it be (against) a relative." [Sûrah al-An`âm
He also says: "O you who believe! stand out firmly for Allah, as witnesses to fair dealing, and let not the hatred of any people make you swerve to wrong and depart from justice. Be just: that is next to piety: and fear Allah. For Allah is well-acquainted with all that you do. " [Sûrah al-Mâ'idah
It is wrong for a person to accuse anyone else of something wrong except with full knowledge and tangible proof. It is forbidden to base a judgment against someone on hearsay, conjecture or suspicion.
Allah says: "O you who believe! If a wicked person comes to you with any news, ascertain the truth, lest you harm people unwittingly, and afterwards become full of repentance for what you have done." [Sûrah al-Hujurât:
He also warns us: "O you who believe! Shun much suspicion; for lo! some suspicion is a sin." [Sûrah al-Hujurât
In those cases where one is compelled to mention another person's faults, it is best to mention that person's good points as well. It is wrong to exaggerate the importance of the person's error or stress the fault too much, especially if it is possible that the error was an honest mistake or in a matter where the truth is not 100% clear.
If a person's error is clearly manifest and established by solid evidence, then it is not wrong to warn people against the error and clarify the truth. However, that correction must be carried out appropriately, in a gentle manner that does not drive people away. The mistake itself should be corrected without delving into anything beyond that. For example, the person who made the mistake should not be accused of having bad intentions or an evil motive.
Al-Dhahabî, speaking about the mistakes of the scholars, said the following: [Siyar al-A`lâm al-Nubalâ'
If we were to discredit every person who made mistake in judgment or declare those people as innovators – people who are essentially of good faith and sincerely seek the truth – then scarcely would any of our scholars be spared.
Speaking about Qatâdah – the illustrious Successor – he said [Siyar al-A`lâm al-Nubalâ'
He was one who asserted human free will to the extent of denying Divine Decree over human actions. We ask Allah to protect us from such ideas. On the other hand, no one ever doubted his honesty in the least, or his integrity, or the strength of his memory. It is quite possible that Allah forgives the likes of him for falling into certain innovated ideas with no other intention but to glorify Allah and to hold Him far above (the bad deeds that people commit.) Allah is the judge who is just and merciful towards His servants. He is not to be asked about what He does.
We should keep in mind that people of knowledge, intelligence, integrity, and piety – who are meticulous in investigating matters and who are quite often correct in their conclusions – they are to be pardoned when they err in their judgment on occasion.
Their good qualities and good work should not be dismissed. Of course, we do not follow what is shown to be wrong, regardless of who the mistake came from. However, we do not use it to discredit the good that a person has done.
And Allah knows best.