Allah said:

الَّذِينَ يَتَّبِعُونَ الرَّسُولَ النَّبِىَّ الأُمِّىَّ الَّذِى يَجِدُونَهُ مَكْتُوبًا عِندَهُمْ فِى التَّوْرَاةِ وَالإِنجِيلِ

(Those who follow the Messenger, the Prophet who can neither read nor write whom they find written of with them in the Tawrah and the Injil.)(7:157) They are as certain of this as they are about who their children are, yet they hide it and distort it. They did not believe in it despite its clear evidence. Therefore Allah said:

إِنَّ الَّذِينَ حَقَّتْ عَلَيْهِمْ كَلِمَةُ رَبِّكَ لاَ يُؤْمِنُونَ - وَلَوْ جَآءَتْهُمْ كُلُّ ءايَةٍ حَتَّى يَرَوُاْ الْعَذَابَ الاٌّلِيمَ

(Truly, those, against whom the Word (wrath) of your Lord has been justified, will not believe. Even if every sign should come to them, until they see the painful torment.) meaning they would not believe in a way that they might benefit from that belief. This is when they believe at a time one may not be able to benefit from his belief. An example is when Musa prayed against Fir`awn and his chiefs, saying:

رَبَّنَا اطْمِسْ عَلَى أَمْوَلِهِمْ وَاشْدُدْ عَلَى قُلُوبِهِمْ فَلاَ يُؤْمِنُواْ حَتَّى يَرَوُاْ الْعَذَابَ الاٌّلِيمَ

(Our Lord! Destroy their wealth, and harden their hearts, so that they will not believe until they see the painful torment.)(10:88) And Allah said:

وَلَوْ أَنَّنَا نَزَّلْنَآ إِلَيْهِمُ الْمَلَـئِكَةَ وَكَلَّمَهُمُ الْمَوْتَى وَحَشَرْنَا عَلَيْهِمْ كُلَّ شَىْءٍ قُبُلاً مَّا كَانُواْ لِيُؤْمِنُواْ إِلاَّ أَن يَشَآءَ اللَّهُ وَلَـكِنَّ أَكْثَرَهُمْ يَجْهَلُونَ

(And even if We had sent down unto them angels, and the dead had spoken unto them, and We had gathered together all things before their very eyes, they would not have believed, unless Allah willed, but most of them behave ignorantly.) (6:111) Allah then said: