The fact that Yahweh's word came to Zephaniah during Josiah's reign (640-609 B.C.) means that he could not have ministered to the Northern Kingdom because it fell in 722 B.C. Thus Zephaniah's audience consisted of the people of Judah, the surviving southern kingdom.
The political situation in Judah during Josiah's reign was fairly peaceful. Following Assyria's capture of Samaria in 722 B.C., the Assyrian Empire began to decline. With its decline, Nabopolassar, the first of the Neo-Babylonian kings (626-605 B.C.), began to push Babylonia forward. Assyria declined and Babylonia advanced until Babylonia, with the Medes and Scythians, destroyed Nineveh in 612 B.C. and a few years later replaced Assyria as the dominant power in the ancient Near East. This happened in 605 B.C. when the Babylonians defeating the Assyrians and Egyptians at Carchemish. Judah benefited during this transitional period in Near Eastern politics. Josiah was able to get rid of some Assyrian religious practices, and he extended Judah's territory north into Naphtali.4
Josiah's evil predecessors, Manasseh (695-642 B.C.) and Amon (642-640 B.C.), had encouraged the people of Judah to depart from the Lord for over 50 years, so wickedness had become ingrained in them. In the eighteenth year of Josiah's reign (622 B.C.) Hilkiah the priest discovered the Law of Moses in the temple, and when Josiah read it he instituted major reforms throughout Judah. Josiah's reforms were good because they were official. He eliminated much of the display of idolatry in the land and revived the celebration of the Passover, among other things.5But unfortunately his reforms did not change the hearts of most of the people, as Jeremiah revealed in his earlier prophecies. So the people to whom Zephaniah ministered had a long history of formal religion without much real commitment to Yahweh.
God sent a prophetic word to Zephaniah because the Judeans of his day still needed to get right with Him in their hearts. The prophet announced that God was going to send judgment on Judah for her wickedness. He also assured the godly few in the nation, the remnant, that the Lord would preserve them and remain true to His promises concerning ultimate worldwide blessing for Israel in the future. Perhaps 1:7 summarizes what the book is all about better than any other single verse: "Be silent before the Lord God! For the day of the LORD is near."
". . . Zephaniah's purpose was to announce coming judgment on Judah in the Day of the Lord. However, he said that judgment would extend to all the nations of the earth, indicating that the Day of the Lord would also bring deliverance for Israel and the Gentiles."6