According to Augustine of Hippo, the term 'angel' refers to "the name of their office, not [...] their nature", as they are pure spirits who act as messengers, clarifying: "If you seek the name of their nature, it is 'spirit'; if you seek the name of their office, it is 'angel': from what they are, 'spirit', from what they do, 'angel."[ Gregory of Nazianzus thought that angels were made as "spirits" and "flames of fire", following Hebrews 1, and that they can be identified with the "thrones, dominions, rulers and authorities" of Colossians 1.
By the late 4th century, the Church Fathers agreed that there were different categories of angels, with appropriate missions and activities assigned to them. There was, however, some disagreement regarding the nature of angels. Some argued that angels had physical bodies, while others maintained that they were entirely spiritual. Some theologians had proposed that angels were not divine but on the level of immaterial beings subordinate to the Trinity. The resolution of this Trinitarian dispute included the development of the doctrine about angels.
Forty Gospel Homilies by Pope Gregory I (c. 540 – 12 March 604) noted angels and archangels. The Fourth Lateran Council's (1215) Firmiter credimus decree (issued against the Albigenses) declared that angels were created beings and men were created after them. The First Vatican Council (1869) repeated this declaration in Dei Filius, the "Dogmatic constitution on the Catholic faith".
Thomas Aquinas (13th century) relates angels to Aristotle's metaphysics in his Summa contra Gentiles, Summa Theologica, and in De substantiis separatis, a treatise on angelology. Although angels have greater knowledge than men, they are not omniscient, as Matthew 24:36 points out. According to the Summa Theologica, angels were created instantaneously by God in a state of grace in the Empyrean Heaven (LXI. 4) at the same time when he created all the contents of the corporeal world (LXI. 3). They are pure spirits whose life consists in knowledge and love. Being bodiless, their knowledge is intellectual and not through senses (LIV. 5). Differently from humans, their knowledge is not acquired from the exterior world (having acquired all knowledge they would ever receive in the moment of their creation); moreover, they attain to the truth of a thing at a single glance without the need of reasoning (LV. a; LVIII. 3,4). They know all that passes in the external world (LV. 2) and the totality of creatures. Still they don't know human' secret thoughts that depend on human free will and thereby are not necessarily linked up with external events (LVII. 4). They don't know the future unless God reveals it to them (LVII. 3).
According to Aquinas, angels are the closest creatures to God. Therefore, like God, they are constituted by pure form without matter. While they do not have a physical composition of matter and form (called ilemorphysm), they possess the metaphysical composition of the act (the act of being) and potency (their finite essence, yet without being. Each angel is a species to which a unique individual belongs to angels differ one from another by way of their unique and irrepetible form. In other words, form - and not matter - is their principle of individuation