John Gill Commentary 2 Chronicles 29:5

"and said unto them, Hear me, ye Levites; now sanctify yourselves, and sanctify the house of Jehovah, the God of your fathers, and carry forth the filthiness out of the holy place." — 2 Chronicles 29:5 (ASV)

And said unto them, hear me, you Levites; sanctify yourselves ,
&c.] By washing their bodies and their garments, that no pollution might attend them in their service, see (Isaiah 52:11)

and sanctify the house of the Lord God of your fathers ;
by carrying out of it all manner of uncleanness, as they did, (2 Chronicles 29:16) ,

and carry forth the filthiness out of the holy place ;
or the abomination; the altar, like that at Damascus, which Ahaz, removing the altar of the Lord, had placed in the court of the priests; or, as Kimchi thinks, an idol, which he had set upon that altar.