In the early stage, the Christian concept of an angel characterized the angel as a messenger of God. Later came identification of individual angelic messengers: Gabriel, Michael, Raphael, and Uriel. Then, in the space of little more than two centuries (from the 3rd to the 5th) the image of angels took on definite characteristics both in theology and in art.
According to St. Augustine, "'Angel' is the name of their office, not of their nature. 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'."