وقالوا لولا أنـزل عليه ملك ولو أنـزلنا ملكا لقضي الأمر ثم لا (6:8) بالانجليزي

(6:8) They also say: 'Why has no angel been sent down to this Prophet?' *5 Had We sent down an angel, the matter would surely have long been decided and no respite would have been granted them. *6

Tafseer Abu-al-A'la Maududi

Page 128 English Traditional

*5). The unbelievers were saying that if Muhammed (peace be on him) had indeed been endowed with prophethood, an angel should have been sent down from heaven to announce that he was the Messenger of God, and that that people would be punished if they did not follow his directives. It was atonishing to these ignorant objectors that the Creator of the heavens and the earth should appoint someone as His Messenger and then leave him without protection against the physical hurts and insults that were flung at him. They would have expected the envoy of a Sovereign as great as the Creator of the universe to be accomplied by at least some heavenly constable, if not a large retinue, to protect him!
*6). The unbelievers' objection is refuted by this remark. it warns them that the time available for accepting the true faith and bringing about the required reform in their lives will last only as long as Ultimate Reality remains concealed from human perception by the wheel of the Unseen. Once that wheel is removed, there will be no more respite: the summons to God's rekoning will have come. This earthly life is a test of whether a man is able to recognize the Ultimate Reality - even though it lies hidden from his sence perception - by the correct excecise of his reason and intelect and of whether, having once recognized it, he is able to behave in conformity with that Reality by exercising control over his animal self and its lusts. This test necessarily requires that the Reality should remain concealed. Thereafter man will be confronted with the result of the test rather than by any further test. Until God has decided to bring the term of their test to a close, it is not possible to respond to such requests by sending angels to them in their true form. (See also Towards Understanding the Qur'an, vol. I, Surah 2, n.228.)