The message of the book of Zephaniah is—you guessed it—judgment and salvation. Judgment will come on the whole earth in general, and on Judah and the nations around her in particular. The people of Judah had given themselves to idolatry (1:4), to half-hearted worship of Yahweh (1:5), and to functional atheism (1:6). Contrary to what they thought (“The Lord will do nothing, either good or bad,”–1:12b), the Lord was active and was going to bring about the day of the Lord (1:14-18).
The prophet issues a call to repentance, urging the people to seek the Lord. Why on earth would they want to seek the one who was going to bring great distress on them? Because only He could help them.
The book closes in hope, with a promise of restoration for the remnant. They are exhorted to rejoice (3:14), even as Yahweh rejoices over them (3:17). Part of this oracle has already been fulfilled in the Lord Jesus’ death on the cross, and we eagerly await its completion on the coming Day.
Sources: Mark Dever, Bob Fyall.