We can have a sense that when we try to do right by one another, we aren't merely striving against ourselves. The feeling is that we are struggling against something--someone-else. As if there's a force-a person- that wishes us ill. In his letter to the Romans, the apostle Paul describes just such a person: Sin, a cosmic tyrant who constrains our moral freedom, confuses our moral judgment, and condemns us to slavery and to death.

Commentators have long argued about whether Paul literally means to say Sin is a person or is simply indulging in literary personification, but regardless of...