What does Luke 12:39 mean?
Jesus switches metaphors. He has explained that those who follow Him well will patiently and expectantly wait for His arrival (Luke 12:35–38). Now, He changes the scene to emphasize that His followers need always to be prepared because they cannot know when He will return.Bible scholars are torn as to the specifics of the characters in this parable. Some say that the "master of the house" is the owner. The parable, then, is a general warning like Luke 12:35, and just says that Jesus' followers need to "stay dressed for action" with "lamps burning."
Others say the "master of the house" represents Jesus' followers, specifically, the "faithful and wise manager, whom his master will set over his household" (Luke 12:42). This interpretation is more likely to identify the thief with Jesus. The purpose of that connection would simply be that Jesus' followers don't know exactly when He is returning, so they need to be constantly vigilant.
Today, the latter fits well, as Jesus' followers are to be watchful for His return (Luke 12:40; Matthew 24:42–44). At the time, the disciples may have taken it as the former, since they had no idea that Jesus would leave and return. To them, the thief may have represented Jesus overthrowing the Roman occupation and making Israel a free nation again.
The King James Version uses "goodman of the house" to mean the "master of the house:" the homeowner or husband.