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Surah 6. Al-An'am
Ayah 7 - 9
٧. وَلَوْ نَزَّلْنَا عَلَيْكَ كِتَـٰبًا فِى قِرْطَاسٍ فَلَمَسُوهُ بِأَيْدِيهِمْ لَقَالَ ٱلَّذِينَ كَفَرُوٓا۟ إِنْ هَـٰذَآ إِلَّا سِحْرٌ مُّبِينٌ
٨. وَقَالُوا۟ لَوْلَآ أُنزِلَ عَلَيْهِ مَلَكٌ ۖ وَلَوْ أَنزَلْنَا مَلَكًا لَّقُضِىَ ٱلْأَمْرُ ثُمَّ لَا يُنظَرُونَ
٩. وَلَوْ جَعَلْنَـٰهُ مَلَكًا لَّجَعَلْنَـٰهُ رَجُلًا وَلَلَبَسْنَا عَلَيْهِم مَّا يَلْبِسُونَ
[7-9] O Messenger, even if We had sent down to you a Book written on paper, and even if they had touched it with their own hands, the disbelievers would have said, "This is nothing but manifest sorcery." And they say, "Why has no angel been sent down to him?"5 If We had sent down an angel, their fate would have been decided long before this and they would have been given no respite after this;6 for if We had sent an angel, We would have sent him in human form and thus involved them in the same doubt in which they are involved now.7
5The ignorant objectors argued that if Muhammad (Allah's peace be upon him) was really a Messenger from Allah, an angel ought to have come down from heaven to declare to the people: "This is the Messenger of Allah, so obey him; otherwise you shall be punished".
The ignorant objectors did not understand how the Creator of the heavens and earth could leave His Messenger in such a plight as to be maligned and stoned by his enemies. They argued that the Messenger of such a Supreme Sovereign ought to have come with a large retinue or at least an angel in attendance on him, to protect him from his enemies and to fill the people with awe and to convince them of his Prophethood and to carry out his orders in some supernatural ways.
6The first answer to their objection is that in case Allah had sent down an angel, no respite would have been given them for reforming themselves and mending their ways. They have been given a chance because no angel has been sent to make the Reality so naked as to leave no alternative for them but to believe. It is obvious that this would have defeated the very purpose of their life in the world to undergo their test. That is why no angel has been sent. Man should undergo his test and discover the unseen Reality without actually seeing it, merely by the right use of his reasoning and thinking powers, and then bring under control his own self and its lusts in accordance with the dictates of that Reality. It is thus obvious that the "Unseen" must remain unseen for the sake of that test.
The worldly life, the period of that test, will remain so only as long as the "Unseen" remains unseen. No sooner will the "Unseen" become evident than that "Period" will automatically come to an end, and instead of test, it will be the time of the result of the test. Therefore Allah is not granting your demand for an angel to appear before you because Allah does not like to end the term of your test before the expiry of the period of the test. (Refer also to E.N. 228, Al-Baqarah).
7The only other alternative was to send an angel in human form. Allah says that if He had sent an angel in human form, they would have felt the same difficulty in recognizing him as they were feeling in recognizing Muhammad (Allah's peace be upon him). This, too, would have involved them in doubts. Therefore it was for their own good that Allah had not sent an angel in attendance on His Messenger.