When Ahlul Sunnat consider the three Caliphs to be the rightful successors of the Prophet, it is necessary to consider them Imams and leaders and not only religious and faithful persons. Presently, according to the opinion of Ahlul Sunnat the three Caliphs are superior to all the Muslims. But we have no argument with the belief of Ahlul Sunnat.
Here the argument is whether people of Shiite faith, believe that the Caliphs were infidels? The belief of Ahlul Sunnat about this is that according to Shias the three Caliphs were infidels. But seeing the reference from Shia books, we come to know that Shias consider them Muslim and not infidels.
According to Majalisul Mo-mineen1 of Qadi Nurullah Shushtari, the great scholar Khwaja Nasiruddin Muhammad Tusi says in his Tajreed that those who fight against Ali (a.s.) are infidels and those who oppose Ali (a.s.) are transgressors. The three Caliphs did not fight against Ali (a.s.), so they cannot be infidels; and when they were not infidels, they were Muslims. According to Allamah Qaushiji in Sharh Tajrid2, people of Shiite faith are not convinced of the infidelity of the three Caliphs. Therefore, it is clear from both the books of Shia and Sunni that according to Shias, the three Caliphs were Muslims.