Shirk Of Ahl-ul-Kitab After Itmam-e-Hujjat And The Definition Of Ahl-ul-Kitab
We can understand this matter in a simple way: When the Prophet SAAWS started his mission, the community got divided into two groups who were identified as believers and disbelievers. When the Jews came into picture, there was a need to identify two groups of disbelievers. For that reason, those who had adopted Shirk as their deen were called Mushrik, and those who believed in the earlier books and claimed to believe in one God, were called Ahlul-kitaab. So the community was identified as three groups: Believers (Muslims) and two groups of disbelievers (Mushrik and Ahlul-kitaab) so that the Qur’aanic guidance could be directed to each group specifically according to their needs.
The following verse indicates the titles of the religious groups at the time of revelation of Qruan, who were known by these titlles, and Quran called them with same names. 22:17 إِنَّ الَّذِينَ آمَنُوا وَالَّذِينَ هَادُوا وَالصَّابِئِينَ وَالنَّصَارَىٰ وَالْمَجُوسَ وَالَّذِينَ أَشْرَكُوا إِنَّ اللَّهَ يَفْصِلُ بَيْنَهُمْ يَوْمَ الْقِيَامَةِ ۚ إِنَّ اللَّهَ عَلَىٰ كُلِّ شَيْءٍ شَهِيدٌ The titles Mushrik and Kafir were not derogative to the holders of thes titles. They owned them. They confessed that they practice Shirk (associating partners with God) 6: 148 سَيَقُولُ الَّذِينَ أَشْرَكُوا لَوْ شَاءَ اللَّهُ مَا أَشْرَكْنَا وَلَا آبَاؤُنَا وَلَا حَرَّمْنَا مِن شَيْءٍ So, it is obvious that they were Mushrik and when Quran called them Mushrik, they did not mind it, otherwise their objection might have been reported. It was a matter of fact. The same goes when they were called kafir, because they did deny the message and they were ok with it: وَقَالُوا إِنَّا كَفَرْنَا بِمَا أُرْسِلْتُم بِهِ In the same sense, Muslims were Kafir to them and Muslim owned it: قدْ كَانَتْ لَكُمْ أُسْوَةٌ حَسَنَةٌ فِي إِبْرَاهِيمَ وَالَّذِينَ مَعَهُ إِذْ قَالُوا لِقَوْمِهِمْ إِنَّا بُرَآءُ مِنكُمْ وَمِمَّا تَعْبُدُونَ مِن دُونِ اللَّهِ كَفَرْنَا بِكُمْ The people of book were called with all the titles they assume for themseves: Bani Israel, Yahoodi, and Ahl e Kitab. Ahl e Kitab or the people of book is an abvious thing. Even if they were not called by this title, this was a matter of distinction for them to be called with the name of the people of Book. That is why they never objected to these titlels, they were being addressed with and none of religious groups ever got confused by the titles that who this title adress to. However, Ahl e Kitab were not ready to be called Mushrik. Therefore they were not called Mushrik.