Charles Ellicott Commentary 2 Chronicles 29:15

"And they gathered their brethren, and sanctified themselves, and went in, according to the commandment of the king by the words of Jehovah, to cleanse the house of Jehovah." — 2 Chronicles 29:15 (ASV)

They gathered their brethren. — As chiefs, or heads of houses, they had the requisite authority. The families mostly concerned would naturally be those residing in Jerusalem.

According to the commandment of the king, by the words of the Lord — that is, through the words of Jehovah; a mandate based on the words of Jehovah, as recorded in the written Law. Compare 1 Chronicles 25:5, and 2 Chronicles 30:12. Also 2 Chronicles 29:25, below: For by the hand of Jehovah was the commandment (Note).