Hadrat [1] Imam-i Rabbani says in the 17th letter of the third volume of his Maktubat:

Angels are the precious born servants of Allahu ta'ala. Some of them have been honoured with conveying information to other angels and to His prophets (alaihimussalam) among mankind. They do what they are commanded to do. They do not revolt. They do not eat or drink. They do not get married. They are neither male nor female. They do not have children. They carried the (heavenly) books and pages. Because they are trustworthy, what they convey is true.

To be a Muslim, it is necessary to believe in angels in this manner. According to the majority of the savants of the right way, the exalted ones from among human beings are superior to the exalted ones from among the angels. Since human beings struggle against the Devil and their own nafs [2], they are exalted, though they are in need. But angels were created exalted. Angels say tasbih [3] and taqdees [4]. Yet it is peculiar to the higher ones from among human beings to add jihad to it. Allahu ta'ala declares in the ninety-fourth ayat [5] of Surat-un-Nisa [6], as interpreted: "Those Muslims who perform jihad, who war against the enemies of religion by sacrificing their possessions and lives for Allah's sake, are more exalted than those who do not go out, but instead only worship. I promise Paradise for all of them."

[1] Hadrat: title of respect used before the names of great people like and Islamic scholars.
[2] nafs: a force in man which wants him to harm himself religiously; an-nafs al-ammara. A negative force within man prompting him to do evil. (Nafs-i ammara). Nafs is ammara by creation, that is, it always wishes evil and harmful deeds to be done. It is reluctant to obey the Shari'at. The nafs of a man who obeys the Shari'at and makes progress in the way of tasawwuf becomes mutmainna. It wishes to obey the Shari'at.
[3] tasbih: may be used for the word taqdees, though there is a very subtle difference between their meanings.
[4] taqdees: to declare to be without defects or faults.
[5] ayat: A verse of al-Qur'an al-karim; al-ayat al-karima.
[6] suura(t): a Qur'anic chapter [a chapter of the Qur'an].