The Imam Abu Hanîfah, may God bless him, said: « There is no faith for a person without Islam nor Islam for a person without faith, they are linked to each other as are the face and the reverse of the same thing. » So the believer is the Muslim and the Muslim is the believer.
Allâh ta`âlâ said:
﴿ وَمَن يَبْتَغِ غَيْرَ الإِسْلاَمِ دِينًا فَلَن يُقْبَلَ مِنْهُ وَهُوَ فِي الآخِرَةِ مِنَ الْخَاسِرِينَ ﴾
which means: « the one who takes a religion other than Islam, it will not be accepted from him and he will be among the losers in the Hereafter. » [Sûrat ‘Ali `Imrân, ‘âyah 85]
Allâh Ta`âlâ said in the holy Qur’ân:
﴿ قُلْ يَا أَهْلَ الْكِتَابِ لِمَ تَكْفُرُونَ بِآيَاتِ اللهِ وَاللهُ شَهِيدٌ عَلَى مَا تَعْمَلُونَ ﴾
(Qul yâ ‘ahla l-kitâb lima takfurûna bi ‘âyâti l-Lâhi wa l-Lâhu chahîdun `alâ mâ ta`malûn)
which means: « Say O people of the book why do you disbelieve in the ‘ayah of Allâh while Allâh is witness of what you do. » [Surat ‘Ali `Imran, ‘ayah 98]. The people of the book means those who claim from book and does not mean that they follow the book.
Allâh Ta`âlâ said in the holy Qur’ân:
﴿ وَلَوْ آمَنَ أَهْلُ الْكِتَابِ لَكَانَ خَيْرًا لَّهُم ﴾
(wa law ‘âmana ‘ahlu l-kitâb lakân khayran lahum)
This means: « And if the people of the book had believed it would have been better for them. » [Surat ‘Ali `Imrân, ‘ayah 110]. People of the book are not believers but they claim from the Torah and the ‘injîl and have falsified them.
Allâh ta`âlâ said in the Qur’ân about the people of the book:
﴿ يُحَرِّفُونَ الْكَلِمَ عَنْ مَوَاضِعِهِ ﴾
This means: « They falsify the expressions of the book. » [Sûrat ‘Ali `Imrân, ’ayah 46]
There are among the people of the books who are converted to Islam at the time of the Prophet Muhammad such as the king Negus of Abyssinia who was a Christian then he converted to Islam and became a good Muslim and likewise the scholar of the Jews at Medina Abdu lLah ibnu salam who converted to Islam after asking the Prophet Muhammad Salla lLâhy `alayhi wa sallam three questions. It is these people of the book who converted to Islam that Qur’ân designates as believers as explained in the tafsir of at-Tabariyy and fakhru d-din ar-Râziyy.
See: Islam is the religion of all the Prophets. Unique True Religion
