فَمَا تَسْتَطِيعُونَ صَرْفاً وَلاَ نَصْراً
(then you can neither avert nor find help.) means: they will not be able to avert the punishment from themselves, nor will they be able to help themselves.
وَمَن يَظْلِم مِّنكُمْ
(And whoever among you does wrong,) means by associating others in worship with Allah,
نُذِقْهُ عَذَاباً كَبِيراً
(We shall make him taste a great torment.)
وَمَآ أَرْسَلْنَا قَبْلَكَ مِنَ الْمُرْسَلِينَ إِلاَّ إِنَّهُمْ لَيَأْكُلُونَ الطَّعَامَ وَيَمْشُونَ فِى الاٌّسْوَاقِ وَجَعَلْنَا بَعْضَكُمْ لِبَعْضٍ فِتْنَةً أَتَصْبِرُونَ وَكَانَ رَبُّكَ بَصِيراً
(20. And We never sent before you any of the Messengers but verily, they ate food and walked in the markets. And We have made some of you as a trial for others; will you have patience And your Lord is Ever All-Seer.)
Allah tells us about the previous Messengers He sent: they all used to eat food needing the nourishment in it. They used to go around in the marketplaces seeking to engage in trade and earn a livelihood. This should not, however, affect their status as Messengers, for Allah gave them good characteristics and caused them to speak fine words and do noble deeds, and gave them miracles and clear proofs, from which any person with sound insight may see the confirmation that what they brought from Allah was true. This Ayah is similar to the Ayat;
وَمَآ أَرْسَلْنَا مِن قَبْلِكَ إِلاَّ رِجَالاً نُّوحِى إِلَيْهِمْ مِّنْ أَهْلِ الْقُرَى
(And We sent not before you any but men unto whom We revealed, from among the people of townships) (12:109).
وَمَا جَعَلْنَـهُمْ جَسَداً لاَّ يَأْكُلُونَ الطَّعَامَ
(And We did not create them bodies that ate not food) (21:8).
وَجَعَلْنَا بَعْضَكُمْ لِبَعْضٍ فِتْنَةً أَتَصْبِرُونَ
(And We have made some of you as a trial for others; will you have patience) means, `We test some of you by means of others, so that We may see who will be obedient and who will be disobedient.' Allah says:
أَتَصْبِرُونَ وَكَانَ رَبُّكَ بَصِيراً
(will you have patience And your Lord is Ever All-Seer.) meaning, He knows who deserves to receive revelation, as Allah says elsewhere:
اللَّهُ أَعْلَمُ حَيْثُ يَجْعَلُ رِسَالَتَهُ
(Allah knows best with whom to place His Message) (6:124). And He knows who deserves to be guided to the Message with which He sent them, and who does not deserve to be guided.
وَجَعَلْنَا بَعْضَكُمْ لِبَعْضٍ فِتْنَةً أَتَصْبِرُونَ
(And We have made some of you as a trial for others. Will you have patience) Muhammad bin Ishaq said: Allah is saying, "If I had willed that the world be such that no one would oppose My Messengers, I could have made it so, but I wanted to test My servants by means of them." In Sahih Muslim it is narrated from `Iyad bin Himar that the Messenger of Allah ﷺ said:
«يَقُولُ اللهُ تَعَالى: إِنِّي مُبْتَلِيكَ وَمُبْتَلٍ بِك»
(Allah says: "I will test you and test others by means of you.") In the Sahih it is recorded that he was given the choice between being a Prophet and king, or being a servant and Messenger, and he chose to be a servant and Messenger.