وَقَالُوا امَنَّا بِهِ
And they will say;"We do believe (now);"
means, on the Day of Resurrection, they will say, `we believe in Allah and His angels, Books and Messengers.'
This is like the Ayah;
وَلَوْ تَرَى إِذِ الْمُجْرِمُونَ نَاكِسُواْ رُءُوسِهِمْ عِندَ رَبِّهِمْ رَبَّنَأ أَبْصَرْنَا وَسَمِعْنَا فَارْجِعْنَا نَعْمَلْ صَـلِحاً إِنَّا مُوقِنُونَ
And if you only could see when the criminals shall hang their heads before their Lord (saying);"Our Lord! We have now seen and heard, so send us back that we will do righteous good deeds. Verily, we now believe with certainty." (32;12)
Allah says;
وَأَنَّى لَهُمُ التَّنَاوُشُ مِن مَكَانٍ بَعِيدٍ
but how could they receive from a place so far off!
meaning, how could they attain faith now that they are so far removed from the place where it could be accepted from them, and they have entered the realm of the Hereafter which is the realm of reward and punishment, not the realm of trial and testing. If they had believed in this world, that would have been good for them, but now they have entered the Hereafter, there is no way that their faith can be accepted, just as there is no way a person can get a thing if he is far away from it.
Mujahid said;
وَأَنَّى لَهُمُ التَّنَاوُشُ
but how could they At-Tana'wush means,
How could they attain that?
Az-Zuhri said,
They will wish to attain faith when they have reached the Hereafter and are cut off from this world.
Al-Hasan Al-Basri said,
They will seek something when they have no way of attaining it, they will seek faith from a distant place