John of Damascus — on Ps 104:4 (ORTHODOX FAITH 2:3)

“[God] is the maker and creator of the angels. He brought into being and made them after his own image into a bodiless nature, some sort of spirit, as it were, and immaterial fire—as the divine David says: "Who makes his angels spirits and his ministers a burning fire." And he determined their lightness, fieriness, heat, extreme acuity, their keenness in their desire for God and his service and their being raised up and removed from every material consideration.”