Jesus concluded His instructions concerning the importance of vigilance in view of His return by giving a parable advocating watchfulness.
The introductory "but"connects this illustration with the former one and identifies a contrast. Jesus is like a thief in only one respect, namely that other people will not expect His coming. The point of this parable is that if a homeowner knows the general time when a thief will break in he will prepare accordingly. The signs of the times during the Tribulation that Jesus revealed (vv. 5-22) will enable believers to know the general time He will return. Consequently believers in the Tribulation should prepare themselves.
This concludes the emphasis on vigilance that marks the first part of Jesus' instructions to His disciples anticipating His return and the end of the present age.