Many of the angels in Judaism/Christianity were influenced by Zoroastrianism. That religion describes an epic battle between Ahura Mazda (God) and his angels and his brother, Ahriman (the devil) and his demons.
Islamic ideals also hold that God (Allah) has a contingent of angels at his command.
In each of these religions and others, angels are the messengers of the gods. They provide and relay the services of God to humans on earth. Although they take different forms for different religions (or in some, are not described at all), they perform the same purpose. Angels are the embodiment of each religion's ideals (which, when pared down to the basics, are mostly similar).
What are some of your favorite romances featuring angels?