With all his heart and with all his soul and with all his strength, in accordance with all the Law of Moses

2 Kings 23:15-27

Even the altar at Bethel, the high place made by Jeroboam son of Nebat, who had caused Israel to sin—even that altar and high place he demolished. He burned the high place and ground it to powder, and burned the Asherah pole also. Then Josiah looked around, and when he saw the tombs that were there on the hillside, he had the bones removed from them and burned on the altar to defile it, in accordance with the word of the Lord proclaimed by the man of God who foretold these things.(1 King 13:11)

The people of the city said, “It marks the tomb of the man of God who came from Judah and pronounced against the altar of Bethel the very things you have done to it.”

“Leave it alone,” he said. “Don’t let anyone disturb his bones.” So they spared his bones and those of the prophet who had come from Samaria.

Josiah slaughtered all the priests of those high places on the altars and burned human bones on them. Then he went back to Jerusalem.

The king gave this order to all the people: “Celebrate the Passover to the Lord your God, as it is written in this Book of the Covenant.” Neither in the days of the judges who led Israel nor in the days of the kings of Israel and the kings of Judah had any such Passover been observed. But in the eighteenth year of King Josiah, this Passover was celebrated to the Lord in Jerusalem.

Neither before nor after Josiah was there a king like him who turned to the Lord as he did—with all his heart and with all his soul and with all his strength, in accordance with all the Law of Moses.

The clean-up extends beyond Judah to former Israelite territory. The neglected Feast of Passover is celebrated once more and private malpractices are dealt with.

Why?

For about 300 years God has known the bones of the man of God and Israel prophet their bone will be preserved, didn't disturb by King Josiah. 

Hezekiah has faith in the Lord, and Josiah has good behavior and obeys the word of the Lord, so they and their country were blessings.

How?

No matter what the situation in any generation, when the King obeys the word of the Lord, this kingdom is blessed from ancient until now, it is the truth.