He burnt the bones of the priests - כומריא kumeraiya, the kemarim, says the Targum. See this word explained, 2 Kings 23:5 (note).

And he burnt the bones of the priests upon their altars,.... On which they sacrificed, in detestation of their idolatry, and to deter from it; and this he did according to the prophecy of him, above three hundred and fifty years before:

and cleansed Judah and Jerusalem; from idolatry, and all the monuments of it.

And he burnt the bones of the priests upon their altars, i.e., he caused the bones of the idolatrous priests to be taken from their graves and burnt on the spot where the destroyed altars had stood, that he might defile the place with the ashes of the dead. In these words is summarized what is stated in 2 Kings 23:13 and 2 Kings 23:14 as to the defilement of the places of sacrifice built upon the Mount of Olives by the bones of the dead, and in 2 Kings 23:16-20 as to the burning of the bones of the high priests of Bethel, after they had been taken from their graves, upon their own altars. מזבחותים is an orthographical error for מזבּחותם.

And he burnt the {d} bones of the priests upon their altars, and cleansed Judah and Jerusalem.

(d) Read 2Ki 23:16.

5. he 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.

34:1-33 Josiah's good reign in Judah. - As the years of infancy cannot be useful to our fellow-creatures, our earliest youth should be dedicated to God, that we may not waste any of the remaining short space of life. Happy and wise are those who seek the Lord and prepare for usefulness at an early age, when others are pursuing sinful pleasures, contracting bad habits, and forming ruinous connexions. Who can express the anguish prevented by early piety, and its blessed effects? Diligent self-examination and watchfulness will convince us of the deceitfulness and wickedness of our own hearts, and the sinfulness of our lives. We are here encouraged to humble ourselves before God, and to seek unto him, as Josiah did. And believers are here taught, not to fear death, but to welcome it, when it takes them away from the evil to come. Nothing hastens the ruin of a people, nor ripens them for it, more than their disregard of the attempts made for their reformation. Be not deceived, God is not mocked. The current and tide of affections only turns at the command of Him who raises up those that are dead in trespasses and sins. We behold peculiar loveliness, in the grace the Lord bestows on those, who in tender years seek to know and to love the Saviour. Hath Jesus, the Day-spring from on high, visited you? Can you trace your knowledge of this light and life of man, like Josiah, from your youth? Oh the unspeakable happiness of becoming acquainted with Jesus from our earliest years!