As Jesus' great sermon called the "Sermon on the Mount" progresses, its message grows in pungency and particularity. The soaring Beatitudes that begin this section are surprising in content but so lofty in scope that listeners or readers could avoid finding themselves personally challenged. Jesus' directive about "salt" and "light" begins to personalize his message by evoking the emphatic "you." Yet the call to be "salt" and "light" is sufficiently vague that the audience could remain reasonably comfortable in their seats.

Beginning in verse 17, however, Jesus begins to reveal the meat of his text - and tough and chewy it proves to be. If the crowds thought Jesus' unusual, user-friendly methods indicated some libertarian streak in his teachings, they cringed at this assertion that not even…