What is the ruling on one who declares a Muslim an infidel in his heart without uttering it, and what is the ruling on doubting the disbelief of a Muslim, and does declaring someone an infidel revert to the one who said it?

It is impermissible to declare a Muslim a disbeliever (kafir) unless the conditions for such a declaration are met and no impediments exist. Furthermore, one whose faith has been established with certainty does not lose it due to doubt, but rather through the establishment of proof and the removal of ambiguity. It is essential to beware of declaring Muslims disbelievers due to sins and transgressions; this is a false belief and a poor presumption about a Muslim. The hadith: "Any man who says to his brother, 'O disbeliever,' then one of them has certainly incurred it. If it is as he said, otherwise it returns upon him," serves as a deterrent and a strong warning against declaring Muslims disbelievers. The تحقيق (verification/analysis) is that it was conveyed to deter a Muslim from saying that to his Muslim brother, and its meaning is that the declaration of disbelief returns upon him, or that it is feared that this might lead him to disbelief. Thus, what returns is the declaration of disbelief, not disbelief itself.