John Gill Commentary 2 Kings 21:4

"And he built altars in the house of Jehovah, whereof Jehovah said, In Jerusalem will I put my name." — 2 Kings 21:4 (ASV)

And he built altars in the house of the Lord
In the holy place, as distinct from the courts in the next verse; and these were sacred to the idols of the Gentiles:

of which the Lord said, in Jerusalem will I put my name ;
in the temple there, devoted to his service, called by his name, and where his name was called upon, see (Deuteronomy 12:5) (16:6) and to erect altars to idols here must be very abominable to him.