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Surah 4. An-Nisaa'
Ayah 47 - 48
47يَـٰٓأَيُّهَا ٱلَّذِينَ أُوتُوا۟ ٱلْكِتَـٰبَ ءَامِنُوا۟ بِمَا نَزَّلْنَا مُصَدِّقًا لِّمَا مَعَكُم مِّن قَبْلِ أَن نَّطْمِسَ وُجُوهًا فَنَرُدَّهَا عَلَىٰٓ أَدْبَارِهَآ أَوْ نَلْعَنَهُمْ كَمَا لَعَنَّآ أَصْحَـٰبَ ٱلسَّبْتِ ۚ وَكَانَ أَمْرُ ٱللَّهِ مَفْعُولًا
48إِنَّ ٱللَّهَ لَا يَغْفِرُ أَن يُشْرَكَ بِهِۦ وَيَغْفِرُ مَا دُونَ ذَٰلِكَ لِمَن يَشَآءُ ۚ وَمَن يُشْرِكْ بِٱللَّهِ فَقَدِ ٱفْتَرَىٰٓ إِثْمًا عَظِيمًا
[47-48] O you to whom the Book had been given! Believe in the Book We have sent down now, confirming the Book you already have with you,77 believe in it before We distort faces and set them backwards, or lay Our curse on them as We laid Our curse on the Sabbath breakers:78 and remember that Allah's Command is surely carried into effect. Shirk is the only sin that Allah does not forgive,79 and He forgives whomsoever He pleases, other sins than this,80 for whosoever associates any other partner with Allah, does indeed forge a big lie and commit the most heinous sin.
77Please refer to E.N. 2, AI -i-`Imran.
78Please refer to E.N.'s 82 and 83, Al-Baqarah.
79The people of the Book have been admonished for being guilty of shirk, though they professed to believe in the Prophets and the revealed Books.
80This does not mean that if one refrains from shirk, then one may freely indulge in other sins. This is merely to bring home to them the gravity of this heinous sin, which they regarded as a trivial thing. That is why the warning has been given that shirk is the most heinous of all sins; so much so that this is the only sin for which forgiveness is not possible.
The Jewish Rabbis were very particular in the observance of minor laws;nay, they spent most of their time in measuring and weighing those regulations which were not explicitly mentioned in their Scriptures but were derived from them. On the contrary, they regarded shirk as a trivial matter: they not only themselves were involved in it but they did not try to save their community from .shirk and acts of shirk. Accordingly, they did not consider that there was anything wrong in entering into alliance with mushrik communities and helping them.