A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
2 Chronicles 34:1
ch2 34:1JOSIAH'S GOOD REIGN. (Ch2 34:1-2)
Josiah was eight years old--(See on Kg2 22:1). The testimony borne to the undeviating steadfastness of his adherence to the cause of true religion places his character and reign in honorable contrast with those of many of his royal predecessors.
2 Chronicles 34:3
ch2 34:3HE DESTROYS IDOLATRY. (Ch2 34:3-7)
in the eighth year of his reign--This was the sixteenth year of his age, and, as the kings of Judah were considered minors till they had completed their thirteenth year, it was three years after he had attained majority. He had very early manifested the piety and excellent dispositions of his character. In the twelfth year of his reign, but the twentieth of his age, he began to take a lively interest in the purgation of his kingdom from all the monuments of idolatry which, in his father's short reign, had been erected. At a later period, his increasing zeal for securing the purity of divine worship led him to superintend the work of demolition in various parts of his dominion. The course of the narrative in this passage is somewhat different from that followed in the Book of Kings. For the historian, having made allusion to the early manifestation of Josiah's zeal, goes on with a full detail of all the measures this good king adopted for the extirpation of idolatry; whereas the author of the Book of Kings sets out with the cleansing of the temple, immediately previous to the celebration of the passover, and embraces that occasion to give a general description of Josiah's policy for freeing the land from idolatrous pollution. The exact chronological order is not followed either in Kings or Chronicles. But it is clearly recorded in both that the abolition of idolatry began in the twelfth and was completed in the eighteenth year of Josiah's reign. Notwithstanding Josiah's undoubted sincerity and zeal and the people's apparent compliance with the king's orders, he could not extinguish a strongly rooted attachment to idolatries introduced in the early part of Manasseh's reign. This latent predilection appears unmistakably developed in the subsequent reigns, and the divine decree for the removal of Judah, as well as Israel, into captivity was irrevocably passed.
2 Chronicles 34:4
ch2 34:4the graves of them that had sacrificed unto them--He treated the graves themselves as guilty of the crimes of those who were lying in them [BERTHEAU].
2 Chronicles 34:5
ch2 34:5he burnt the bones of the priests upon their altars--A greater brand of infamy could not have been put on idolatrous priests than the disinterment of their bones, and a greater defilement could not have been done to the altars of idolatry than the burning upon them the bones of those who had there officiated in their lifetime.
2 Chronicles 34:6
ch2 34:6with their mattocks--or, "in their deserts"--so that the verse will stand thus: "And so did [namely, break the altars and burn the bones of priests] he in the cities of Manasseh, and Ephraim, and Simeon, even unto Naphtali, in their deserted suburbs." The reader is apt to be surprised on finding that Josiah, whose hereditary possessions were confined to the kingdom of Judah, exercised as much authority among the tribes of Ephraim, Manasseh, Simeon, and others as far as Naphtali, as he did within his own dominion. Therefore, it is necessary to observe that, after the destruction of Samaria by Shalmaneser, the remnant that continued on the mountains of Israel maintained a close intercourse with Judah, and looked to the sovereigns of that kingdom as their natural protectors. Those kings acquired great influence over them, which Josiah exercised in removing every vestige of idolatry from the land. He could not have done this without the acquiescence of the people in the propriety of this proceeding, conscious that this was conformable to their ancient laws and institutions. The Assyrian kings, who were now masters of the country, might have been displeased at the liberties Josiah took beyond his own territories. But either they were not informed of his doings, or they did not trouble themselves about his religious proceedings, relating, as they would think, to the god of the land, especially as he did not attempt to seize upon any place or to disturb the allegiance of the people [CALMET].
2 Chronicles 34:8
ch2 34:8HE REPAIRS THE TEMPLE. (Ch2 34:8-18)
in the eighteenth year of his reign . . . he sent Shaphan--(See on Kg2 22:3-9).
2 Chronicles 34:19
ch2 34:19AND, CAUSING THE LAW TO BE READ, RENEWS THE COVENANT BETWEEN GOD AND THE PEOPLE. (Ch2 34:19-33)
when the king had heard the words of the law, &c.--(See on Kg2 22:11-20; Kg2 23:1-3).