Commentary for II Chronicles 29:34

Only the priests were few i.e., Those who had consecrated themselves were still few; so their kinsmen, the Levites, reinforced them until the work was completed and until the priests consecrated themselves. Now, should you ask: How was it that the Levites consecrated themselves and the priests did not consecrate themselves? Scripture explains: “for the Levites were more conscientious than the priests to consecrate themselves”; the Levites were more conscientious than the priests, for the priests and the Levites and all God-fearing people had disguised themselves and made themselves appear unrecognizable, and had even hidden, all the days of the wicked kings, and when Hezekiah - who was righteous - came, they could not immediately consecrate themselves and cleanse themselves, for this was in the first year of his reign; so they came and consecrated themselves gradually, as it is written (Prov. 28:12): “When the righteous rejoice, there is much glory, but when the wicked rise up, a person disguises himself,” and it is written (ibid. verse 28): “When the wicked rise up, people hide; but when they perish, the righteous increase.” When the wicked Ahaz rose up, people hid, but when he perished, the righteous increased, i.e., Hezekiah and his company.