He makes a comparison to the unexpectedness of a thief who comes into your house at night and steals something. If you had known, you would have been sitting there with your gun or have the police sitting there with you. Believers are not caught off guard by the thief because we are not in the night-time period. You have to understand the concept of the Jewish day. The Jewish day started with in the evening and then came the daylight.
Tribulation comes before blessing.
Therefore, when Thess says you are not of the night, it means they would not be in the tribulation. And if the thief comes in the night, he is coming in the tribulation. The point of the Noah and Lot illustrations is the judgment. Those that were not ready for the flood were all destroyed. Same in Sodom and Gomorrah.