Is it permissible to warn against those who excessively declare rulers to be disbelievers (takfiris), and is it considered backbiting (ghiba) if one says, "So-and-so is a takfiri," with the intention of warning?

One should not proceed to declare a specific individual an infidel unless the conditions are met and the impediments are removed. Issues of faith and disbelief should be discussed in the way that accredited scholars have stated them, for these are grave matters, and delving into them ignorantly is a source of misguidance. One should not rush to label a person as an innovator (mubtadi') or describe them as a takfiri or a Kharijite unless their adherence to innovation has been verified, for perhaps the matter is one about which scholars have differed, in which case the person would fall into backbiting and injustice. If it is verified that someone is an innovator and there is fear that people might be influenced by their innovation, then it is permissible to warn against them.