Why do you not bring angels to us if you are of the truthful!
i.e., to bear witness to the accuracy of what you have brought to us is true, if you are really telling the truth!
This is similar to what Pharaoh said;
فَلَوْلَا أُلْقِىَ عَلَيْهِ أَسْوِرَةٌ مِّن ذَهَبٍ أَوْ جَأءَ مَعَهُ الْمَلَـيِكَةُ مُقْتَرِنِينَ
Why then are not golden bracelets bestowed on him, or angels sent along with him. (43;53)
And Allah said;
وَقَالَ الَّذِينَ لَا يَرْجُونَ لِقَأءَنَا لَوْلَا أُنزِلَ عَلَيْنَا الْمَلَـيِكَةُ أَوْ نَرَى رَبَّنَا لَقَدِ اسْتَكْبَرُواْ فِى أَنفُسِهِمْ وَعَتَوْا عُتُوّاً كَبِيراً
يَوْمَ يَرَوْنَ الْمَلَـيِكَةَ لَا بُشْرَى يَوْمَيِذٍ لِّلْمُجْرِمِينَ وَيَقُولُونَ حِجْراً مَّحْجُوراً
And those who do not expect a meeting with Us (i.e., those who deny the Day of Resurrection and the life of the Hereafter), say;"Why are not the angels sent down to us, or why do we not see our Lord!"
Indeed they think too highly of themselves, and are scornful with great pride.
On the Day that they do see the angels - there will be no good news given on that day to the guilty. And they (angels) will say;"All kinds of glad tidings are forbidden for you." (25;21-22)
For this reason Allah said;
مَا نُنَزِّلُ الْمَليِكَةَ إِلاَّ بِالحَقِّ وَمَا كَانُواْ إِذًا مُّنظَرِينَ