بَلْ بَدَا لَهُم مَّا كَانُواْ يُخْفُونَ مِن قَبْلُ
Nay, what they had been concealing before has become manifest to them.
meaning, the disbelief, denial and rebellion that they used to hide in their hearts will then be uncovered, even though they will try to hide this fact in this life and the Hereafter.
Earlier, Allah said,
ثُمَّ لَمْ تَكُن فِتْنَتُهُمْ
إِلاَّ أَن قَالُواْ وَاللّهِ رَبِّنَا مَا كُنَّا مُشْرِكِينَ
انظُرْ كَيْفَ كَذَبُواْ عَلَى أَنفُسِهِمْ وَضَلَّ عَنْهُم مَّا كَانُواْ يَفْتَرُونَ
There will then be (left) no trial for them but to say;"By Allah, our Lord, we were not those who joined others in worship with Allah." Look! How they lie against themselves! But the (lie) which they invented will disappear from them. (6;23-24)
It is also possible that the meaning here is that the disbelievers will realize the truth that they knew all along in their hearts, that is, that what the Messengers brought them in this life is true, although they used to deny his Message before their followers.
Allah said that Musa said to Fir`awn,
لَقَدْ عَلِمْتَ مَأ أَنزَلَ هَـوُلاءِ إِلاَّ رَبُّ السَّمَـوَتِ وَالاٌّرْضِ بَصَأيِرَ
"Verily, you know that these signs have clearly been sent down by none but the Lord of the heavens and the earth as eye-opening evidence." (17;102)
Allah said about Fir`awn and his people,
وَجَحَدُواْ بِهَا وَاسْتَيْقَنَتْهَأ أَنفُسُهُمْ ظُلْماً وَعُلُوّاً
And they belied them (those Ayat) wrongfully and arrogantly, though they were themselves convinced thereof. (27;14)
بَلْ بَدَا لَهُم مَّا كَانُواْ يُخْفُونَ مِن قَبْلُ
Nay, it has become manifest to them what they had been concealing before.
When this occurs, and the disbelievers ask to be returned to this life, they will not do so because they truly wish to embrace the faith. Rather, they ask to be returned to this life for fear of the torment that they are witnessing before them, as punishment for the disbelief they committed, and to try and avoid the Fire that they see before their eyes.
وَلَوْ رُدُّواْ لَعَادُواْ لِمَا نُهُواْ عَنْهُ وَإِنَّهُمْ لَكَاذِبُونَ
But if they were returned, they would certainly revert to that which they were forbidden. And indeed they are liars.
meaning, they lie when they say they wish to go back to this life so that they can embrace the faith.
Allah states that even if they were sent back to the life of this world, they will again commit the disbelief and defiance that they were prohibited.
وَإِنَّهُمْ لَكَاذِبُونَ
(And indeed they are liars), in their statement that,
فَقَالُواْ يَا لَيْتَنَا نُرَدُّ وَلَا نُكَذِّبَ بِأيَاتِ رَبِّنَا وَنَكُونَ مِنَ الْمُوْمِنِينَ
بَلْ بَدَا لَهُم مَّا كَانُواْ يُخْفُونَ مِن قَبْلُ
وَلَوْ رُدُّواْ لَعَادُواْ لِمَا نُهُواْ عَنْهُ وَإِنَّهُمْ لَكَاذِبُونَ
"Would that we were but sent back! Then we would not deny the Ayat of our Lord, and we would be of the believers!"
Nay, what they had been concealing before has become manifest to them. But if they were returned, they would certainly revert to that which they were forbidden. And indeed they are liars.
Therefore, they will revert to their old behavior