Scripture's Terrifying Angels and the Paradox

When an angel shows up in scripture, its first words are often "be not afraid." Who are these creatures, and what role do they play in Christian doctrine and tradition? Part of a Constant Wonder series marking the Christian Advent with daily vignettes that distill the spirit of the season, in keeping with our mission of exploring wonder and awe in all nature, human or wild, vast or small: encounters that move us beyond words. Guest: Ed Simon, author of "Elysium: A Visual History of Angelology"