The believers replied to them that after Allah (swt) showed them mercy, granted them guidance, made it clear for them and sent them a Messenger, Prophet Shu’ayb (PBUH), then returning to the false religion they had before would be fabricating lies against Allah (swt).
( وَمَا يَكُونُ لَنَا أَن نَّعُودَ فِيهَا إِلَّا أَن يَشَاءَ اللَّهُ رَبُّنَا) (And it is not for us to return to it except that Allah, our Lord, should will): They will never return to the previous religion they had before, yet they submitted the matter of believing to the will of Allah (swt) because the hearts are as our Prophet Muhammad (saws) described in:
“The hearts of all men are between two of the Compassionate’s fingers as if they were one heart which He turns about as He wills.”[1]
As a matter of fact, no one can assure maintaining his faith, thus it was placed under the condition, which is Allah’s Will (swt).
(except that Allah, our Lord, should will): They will never return to it except by the Will of Allah (swt).
( وَسِعَ رَبُّنَا كُلَّ شَيْءٍ عِلْمًا) (Our Lord has encompassed all things in knowledge): Allah’s Knowledge encompasses everything. He (swt) knows the secrets and what is more hidden. Allah (swt) knows what was, what will be, and what is currently happening.
( عَلَى اللَّهِ تَوَكَّلْنَا ) (Upon Allah we have relied): They strengthened their resolve by placing their trust in Allah (swt), because they were in dire need of a miracle. Otherwise, their people would have expelled them and subjected them to hardship.
( رَبَّنَا افْتَحْ بَيْنَنَا وَبَيْنَ قَوْمِنَا بِالْحَقِّ) (Our Lord, decide between us and our people in truth): (Decide) means to make a decisive judgment. They are invoking Allah (swt) to make a wise judgment between them and the disbelievers. The Arabic text of the verse uses the word which literally means (open). As known, this word is usually pertained to physical closed object. Allah (swt) says:
وَلَمَّا فَتَحُوا مَتَاعَهُمْ وَجَدُوا بِضَاعَتَهُمْ رُدَّتْ إِلَيْهِمْ
(And when they opened their baggage, they found their merchandise returned to them) (Yusuf: 65)
On the other hand , this Arabic word can be used to indicate moral meanings, such as (grant) in the following verse:
مَّا يَفْتَحِ اللَّهُ لِلنَّاسِ مِن رَّحْمَةٍ فَلَا مُمْسِكَ لَهَا
(Whatever Allah grants to people of mercy – none can withhold it) (Faatir: 2)
And in another verse, it means (opened upon them):
وَلَوْ أَنَّ أَهْلَ الْقُرَىٰ آمَنُوا وَاتَّقَوْا لَفَتَحْنَا عَلَيْهِم بَرَكَاتٍ مِّنَ السَّمَاءِ وَالْأَرْضِ وَلَٰكِن كَذَّبُوا فَأَخَذْنَاهُم بِمَا كَانُوا يَكْسِبُونَ
(And if only the people of the cities had believed and feared Allah, We would have opened upon them blessings from the heaven and the earth; but they denied [the messengers], so We seized them for what they were earning.”) (Al-A’raaf: 96)
It can mean (pray for victory) as well, Allah (swt) says:
وَكَانُوا مِن قَبْلُ يَسْتَفْتِحُونَ عَلَى الَّذِينَ كَفَرُوا
(although before they used to pray for victory against those who disbelieved) (Al-Baqara: 89)
[1] Sahih Muslim, the Book of Destiny, no. 2654.