(Let him then bring us an Ayah like the ones (Prophets) that were sent before (with signs)!) 21:5 We mentioned this subject several times before and stated that Allah is able to bring them what they wanted. There is a Hadith which mentions that the idolators asked the Prophet to turn Mount As-Safa into gold and, they also asked him for a spring to gush forth for them and to remove the mountains from around Makkah and replace them with green fields and gardens. Allah revealed to His Messenger : "If You wish, O Muhammad, I will give them what they asked for. However, if they disbelieve thereafter, I will punish them with a punishment that I did not punish any among the `Alamin (mankind and the Jinns). Or, if you wish, I will open for them the door to repentance and mercy." The Prophet said,
At the beginning of the section, human beings were divided into two kinds - those who are obedient to Allah Ta’ ala and those who are disobedient to Him. Then, enumerated there were some attributes and signs of the obedient servants of Allah and mention was made of the best of rewards for them in the Hereafter.
Now, in the present verses, stated there are the attributes and signs, and punishments, of the other kind of people. One trait of character these disobedient and contumacious people have been reported to have الَّذِينَ يَنقُضُونَ عَهْدَ اللَّـهِ مِن بَعْدِ مِيثَاقِهِ (And those who break [ their ] pledge with Allah after it has been made binding). Included here is the pledge out of the pledges given to Allah concerning His unshared Lordship and Oneness by all spirits created by Him. The disbelievers and polytheists broke this pledge when they came into this world and consequently took to hundreds and thousands of lords and objects of worship all joined up with the pristine divinity of Allah Ta` ala.
The reason for the rejection of the preacher of truth is generally that the people do not find any tangible glories around this person. But this amounts to being unsuccessful at the very point where a man has to prove successful. God wants man to recognize the truth in its absolute shape and to surrender himself to that truth. Now, one who insists that he will not accept the Truth without tangible feats will be fated in this world never to find the truth according to the law of God and he will always be deprived of God’s guidance. This world is a testing ground. Here man can find God at the level of remembrance; he cannot find God at the level of ‘observation’. Those who agree with this godly plan will find God and those who do not will be unsuccessful in finding Him, just as one who insists on seeing the sun with his naked eyes will be unsuccessful in doing so. In this world only that person succeeds who accepts God’s plan and who moulds his life according to it, because it is God who is the Creator of this world and not man.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.