Josiah, the son of Amon (a wicked king) was the most godly king since King David. He was also the last good king of Judah. He received the highest assessment: "he did what was right in the sight of the Lord, and walked in the ways of his father David; he did not turn aside to the right hand or to the left" (34:2). Since he was only eight years old when he became king, he must have had some wise and godly counsellors (likely the high priest, as had young Joash, 24:1-2) who instructed him in the ways of the Lord. The parallel account of these chapters is found in 2 Kings 22 and 23. That account gives more details about Josiah's reforms, but the Chronicles account expands upon his observance of the Passover.
When only sixteen, Josiah "began to seek the God of his father David" (34:3). At this point, he must have made it his own personal goal to follow the Lord and study the Law, that he might not sin against God. After four years of seeking the Lord, he began his campaign to purge Jerusalem and all Judah of idolatry. He did not just discard the idols, but pulverized them so they could never be worshipped again. His desire to revive the proper worship of God at the Temple led him to begin a major restoration project when he was twenty-six years old. The faithful remnant from the Northern Kingdom, as well as the people of Judah (including the tribe of Benjamin), contributed with their tithes and offerings to the rebuilding fund. Trusted Levites travelled throughout the land, gathering the donations and tithes, and faithful workers built and crafted everything well, under the oversight of the Levites.
While cleaning out the debris in the Temple, "Hilkiah the priest found the Book of the Law of the Lord given by Moses" (34:14; for further details, see commentary on 2 Kings 22). It may have been an original scroll of Moses that had been kept beside the Ark (Deuteronomy 31:25-26), but had been somehow, misplaced, probably because of the many times the Ark had been moved and the many kings who did not care for God's House. After hearing the words of the holy writings, Josiah was very sorrowful, because he realized how far short they had fallen from keeping the Law and because of the severe punishments God said He would bring upon them for the very transgressions of which they were guilty.
From the prophetess Huldah, Josiah learned that the judgments would indeed come about because of Judah's faithlessness (especially under the reign of Manasseh, 2 Kings 23:26); yet because of his own faithfulness, he would not see them during his lifetime. The words of the Law greatly affected Josiah. He called for a renewal of the covenant, and with all the more fervency, he continued to purge the land of sin and idolatry.
At the newly cleansed and restored Temple, Josiah called for a great Passover celebration, the likes of which had not been seen since the days of Samuel (35:18). It took place on the prescribed day (35:1), unlike Hezekiah's Passover feast which had been delayed by one month. Also, this time the priests and Levites were all prepared and had sanctified themselves ahead of time. Now that the Temple had been cleansed and reconsecrated to the Lord, Josiah ordered that the Ark be put back into its proper place. During the dark periods under the reigns of Ahaz, Manasseh, or Amon, some pious Levites had likely removed the Ark and hidden it in order to protect it from defilement and destruction. Now, however, under the strong and righteous leadership of Josiah, the Ark was safe and once again in its rightful place of honour. The Passover offerings were very numerous, and Josiah himself sacrificed a great number on behalf of the people.
These people were to learn constant dependence upon the Lord for their safety, and it was not His will for them to be involved in the power struggles of Egypt, Babylon, and Assyria. At this time, the Assyrian capital of Nineveh had been captured by the Babylonians. Since Egypt had become an ally of Assyria, Necho king of Egypt and his army were travelling north along the coastal route, through the territory of Judah, to help the Assyrians. Josiah saw it as an opportunity to defeat Egypt, and thus they would pose no threat to Judah's new independence. The Lord even spoke through the heathen king to warn Josiah not to go against him, but Josiah disguised himself (cf. Ahab, 18:29) and went to battle against the will of God. He paid with his life for this lack of trust in God, but God graciously gave him a few hours of life after his mortal injury. During those hours, he must have repented and died at peace with God.