The complaints are certainly very serious and unheard-of, and you will hardly find another example like them. For it is incredible for a man to be so altered and to degenerate not into a beast but into an abortive monstrosity of nature. This is what the common saying means: “When an angel becomes a devil, he really becomes bad.” The devil has become exceedingly corrupt and bad because he has degenerated from a preeminent, angelic nature. Thus a virtuous virgin, after she has once prostituted her virtue, no longer has any respect for anything but casts off all sense of shame for disgraceful acts of every kind. So also, if a Christian becomes an apostate, no other enemy of the church and godliness will be more bitter.
Luther’s works, vol. 6: Lectures on Genesis: Chapters 31-37 p 18