Albert Barnes Commentary 2 Chronicles 34:3

"For in the eighth year of his reign, while he was yet young, he began to seek after the God of David his father; and in the twelfth year he began to purge Judah and Jerusalem from the high places, and the Asherim, and the graven images, and the molten images." — 2 Chronicles 34:3 (ASV)

He began to purge Judah - Jeremiah’s first prophecies (Jeremiah 2–3) appear to have been coincident with Josiah’s earlier efforts to uproot idolatry, and must have greatly strengthened his hands.