Allah reiterated His favor on the Children of Adam when He stated that He mentioned them in the highest of heights before He created them. Allah said,
﴿وَإِذْ قَالَ رَبُّكَ لِلْمَلَـئِكَةِ﴾
(And (remember) when your Lord said to the angels.)
This Ayah means, "O Muhammad ! Mention to your people what Allah said to the angels,
﴿إِنِّي جَاعِلٌ فِى الأَرْضِ خَلِيفَةً﴾
(Verily, I am going to place a Khalifah on earth).
Meaning people reproducing generation after generation, century after century, just as Allah said,
﴿وَهُوَ الَّذِى جَعَلَكُمْ خَلَـئِفَ الاٌّرْضِ﴾
(And it is He Who has made you (Khala'if) generations coming after generations, replacing each other on the earth) (6:165),
﴿وَيَجْعَلُكُمْ حُلَفَآءَ الاٌّرْضِ﴾
(And makes you (Khulafa') inheritors of the earth) (27:62),
﴿وَلَوْ نَشَآءُ لَجَعَلْنَا مِنكُمْ مَّلَـئِكَةً فِى الاٌّرْضِ يَخْلُفُونَ ﴾
(And if it were Our will, We would have (destroyed you (mankind all, and) made angels to replace you (Yakhlufun) on the earth.) (43: 60) and,
﴿فَخَلَفَ مِن بَعْدِهِمْ خَلْفٌ﴾
(Then after them succeeded an (evil) generation (Khalf)) (7:169). It appears that Allah was not refering to Adam specifically as Khalifah, otherwise he would not have allowed the angels' statement,
﴿أَتَجْعَلُ فِيهَا مَن يُفْسِدُ فِيهَا وَيَسْفِكُ الدِّمَآءَ﴾
(Will You place therein those who will make mischief therein and shed blood).
The angels meant that this type of creature usually commits the atrocities they mentioned. The angels knew of this fact, according to their understanding of human nature, for Allah stated that He would create man from clay. Or, the angels understood this fact from the word Khalifah, which also means the person who judges disputes that occur between people, forbidding them from injustice and sin, as Al-Qurtubi said.
The statement the angels uttered was not a form of disputing with Allah's, nor out of envy for the Children of Adam, as some mistakenly thought. Allah has described them as those who do not precede Him in speaking, meaning that they do not ask Allah anything without His permission. When Allah informed them that He was going to create a creation on the earth, and they had knowledge that this creation would commit mischief on it, as Qatadah mentioned, they said,
﴿أَتَجْعَلُ فِيهَا مَن يُفْسِدُ فِيهَا وَيَسْفِكُ الدِّمَآءَ﴾
(Will You place therein those who will make mischief therein and shed blood)
This is only a question for the sake of learning about the wisdom of that, as if they said, Our Lord! What is the wisdom of creating such creatures since they will cause trouble in the earth and spill blood "If the wisdom behind this action is that You be worshipped, we praise and glorify You (meaning we pray to You) we never indulge in mischief, so why create other creatures''
Allah said to the angels in answer to their inquiry,
﴿إِنِّي أَعْلَمُ مَا لاَ تَعْلَمُونَ﴾
(I know that which you do not know.) meaning, "I know that the benefit of creating this type of creature outweighs the harm that you mentioned, that which you have no knowledge of. I will create among them Prophets and send Messengers. I will also create among them truthful, martyrs, righteous believers, worshippers, the modest, the pious, the scholars who implement their knowledge, humble people and those who love Allah and follow His Messengers.''
The Sahih recorded that when the angels ascend to Allah with the records of the servant's deeds, Allah asks them, while having better knowledge, "How did you leave My servants'' They will say, "We came to them while they were praying and left them while they were praying.'' This is because the angels work in shifts with mankind, and they change shifts during the Fajr and `Asr prayers. The angels who descended will remain with us, while the angels who have remained with us ascend with our deeds. The Messenger of Allah said,
«يُرْفَعُ إِلَيْهِ عَمَلُ اللَّيْلِ قَبْلَ النَّهَارِ وَعَمَلُ النَّهَارِ قَبْلَ اللَّيْل»
(The deeds of the night are elevated to Allah before the morning, and the deeds of the morning before the night falls. )
Hence, the angels' statement, "We came to them while they were praying and left them while they were praying,'' explains Allah's statement,
﴿إِنِّي أَعْلَمُ مَا لاَ تَعْلَمُونَ﴾
(I know that which you do not know.)
It was said that the meaning of Allah's statement,
﴿إِنِّي أَعْلَمُ مَا لاَ تَعْلَمُونَ﴾
(I know that which you do not know.) is, "I have a specific wisdom in creating them, which you do not have knowledge of.'' It was also said that it is in answer to,
﴿وَنَحْنُ نُسَبِّحُ بِحَمْدِكَ وَنُقَدِّسُ لَكَ﴾
(While we glorify You with praises and thanks and sanctify You) after which Allah said,
﴿إِنِّي أَعْلَمُ مَا لاَ تَعْلَمُونَ﴾
(I know that which you do not know). Meaning, "I know that Iblis is not as you are, although he is among you.'' Others said,
﴿أَتَجْعَلُ فِيهَا مَن يُفْسِدُ فِيهَا وَيَسْفِكُ الدِّمَآءَ وَنَحْنُ نُسَبِّحُ بِحَمْدِكَ وَنُقَدِّسُ لَكَ﴾
"(Will You place therein those who will make mischief therein and shed blood, ـ while we glorify you with praises and thanks and sanctify You.) is their request that they should be allowed to inhabit the earth, instead of the Children of Adam. So Allah said to them,
﴿إِنِّي أَعْلَمُ مَا لاَ تَعْلَمُونَ﴾
(I know that which you do not know) if your inhabiting the heavens is better, or worse for you.'' Ar-Razi as well as others said this. Allah knows best.