1 Thessalonians 5:2 (ESV)

2 For you yourselves are fully aware that the day of the Lord will come like a thief in the night.

The return of the Lord will happen suddenly—as sudden as the coming of a thief in the night. So it was foretold by Jesus (in Matthew 24:43–44).

The words day of the Lord point to other places in Scripture about the coming of God himself (Amos 5:18). The fact that this relates to the coming of Jesus shows on which godly level Jesus is. In Jesus we are dealing with the mighty God.