Jamieson, Fausset and Brown on Isaiah 57:8

8. "Remembrance," that is, memorials of thy idolatry: the objects which thou holdest in remembrance. They hung up household tutelary gods "behind the doors"; the very place where God has directed them to write His laws "on the posts and gates" (De 6:9; 11:20); a curse, too, was pronounced on putting up an image "in a secret place" (De 27:15).

discovered thyself--image from an adulteress.

enlarged ... bed--so as to receive the more paramours.

made ... covenant--with idols: in open violation of thy "covenant" with God (Ex 19:5; 23:32). Or, "hast made assignations with them for thyself" [Horsley].

thy bed ... their bed--The Jews' sin was twofold; they resorted to places of idolatry ("their bed"), and they received idols into the temple of God ("thy bed").

where--rather, "ever since that" [Horsley]. The Hebrew for "where" means "room" (Margin), a place; therefore, translate, "thou hast provided a place for it" (for "their bed"), namely, by admitting idolatrous altars in thy land [Barnes]; or "thou choosest a (convenient) place for thyself" in their bed [Maurer] (Is 56:5).