'Thief' in the Bible

For you yourselves know perfectly well that the day of the [return of the] Lord is coming just as a thief [comes unexpectedly and suddenly] in the night.

But you, believers, [all you who believe in Christ as Savior and acknowledge Him as God’s Son] are not in spiritual darkness [nor held by its power], that the day [of judgment] would overtake you [by surprise] like a thief;