I shall start with a quotation from Wolf Biermann's “Barlach-Lied”:

Biermann's 1965 song, warning of imminent threat, diagnoses the darkening times by referring to a fall of angels. They fall dead from the heavens like insects or birds in a hostile climate. And similar, if less poetic, obituaries have been issued by a number of commentators in different fields. Gerhard Bott, former Director of the Walraff-Richartz-Museum in Cologne, in an examination of the angel in Western art, speaks of a “Verfall der Engel”: a fall certainly; but he is also pointing to the degeneration of the image. Modern angels have been debased as the result of a long process of vulgarised aesthetic humanisation, socialisation and sexualisation, which began perhaps with the painters of the Italian Renaissance. Paradoxically, as angels became less substantial in eyes of theologians, they became more so in eyes of artists and writers, with the result that in representations they began to obey the laws of nature and science. That process has continued, with every age more or less producing the angels it needs. And, as that has happened, the figure of the angel has fallen to earth — becoming more and more a reflection of the time it represents, and less a reflection of an immutable and divinely sponsored universe.