"We are not of a high order of angels"

You can check out the Christian hierarchy of angels here, though there are further disambiguations in the Muslim, Jewish and Zoroastrian theologies. Our understanding of the hierarchy is comes mainly from The Celestial Hierarchy, a scholarly work of the 4th or 5th century, and the work of scholoars of the Middle Ages. Informed by the New and Old Testament, these works reveal a hierarchy of three spheres, within which are three internal tiers. Each sphere has its own purpose; the first sphere are heavenly protectors, the second are heavenly governers and the third are heavenly messengers or soldiers.

Due to their purpose, the only sphere to have any interaction with man is the third, and this is why we are most familiar with them. The internal tier system is that of rulers, then archangels and finally angels. Though Pullman's hierarchy is not strictly true to this system, it is a worthy aside in understanding the human understanding of the Kingdom of Heaven's mechanisms.