The Position of Ahlul Sunnah regarding Rebellion against the Muslim & Kaafi

The Aqeedah of the Salaf based upon the Quran and the Authentic Sunnah with the understanding of the pious predecessors is that it is impermissible to rebel against the Muslim Ruler, even if he is a tyrant. This is something agreed upon by consensus. However, what happens if the Ruler is a Kaafir, does rebellion become permissible?! The Khawarij/Takfiris when cornered upon the issue of hearing and obeying the Muslim Ruler will spread doubts, saying that if the Ruler is a Kafir then we can reb