Is it permissible for a Muslim to declare his brother an infidel (kafir) and tarnish his reputation, especially if he is inviting people to Allah, and does this apply to those who tarnish the reputation of the preacher Amr Khaled and other preachers?

Making mistakes in some specific points does not render a person an apostate (kafir) or an innovator (mubtadi'), nor does it prevent benefiting from them. Rather, their emulation in what they erred in should be abandoned, with the truth being clarified without diminishing their status. Justice must be observed in judging people and acknowledging the virtue of people of virtue. The Prophet, peace and blessings be upon him, said: "Indeed, part of glorifying Allah is honoring a white-haired Muslim, and a carrier of the Qur'an who does not exceed its bounds nor abandon it, and honoring a just ruler." And he said: "He is not of us who does not respect our elders, show mercy to our young, and acknowledge the right of our scholar." Scholars have consistently weighed good deeds against bad deeds and judged with justice, as Al-Dhahabi and Ibn Taymiyyah stated, citing Allah's saying: "O you who have believed, be persistently Qawwameen for Allah , witnesses in justice, and do not let the hatred of a people prevent you from being just. Be just; that is nearer to righteousness. And fear Allah; indeed, Allah is Acquainted with what you do."