The word of the LORD which came unto Zephaniah the son of Cushi, the son of Gedaliah, the son of Amariah, the son of Hizkiah, in the days of Josiah the son of Amon, king of Judah.
The word of the Lord which came unto Zephaniah ... in the days of Josiah. Had their idolatries been under former kings, they might have said, our kings have forced us to this or that; but under Josiah, who did all in his power to reform them, they have no such excuse.
The son of Amon - the idolater, whose bad practices the Jews clung to, rather than the good example of Josiah, his son, so incorrigible were they in sin. They were desperately ready to go back to idolatry, but sadly slow in going back to the worship of Yahweh, which was the worship of Israel under their best king, David, ages before.
King of Judah. Israel's ten tribes had ere this gone into captivity.