Co -regent? Jesus Christ co- regent? Has man elevated himself to be co-regent with GOD?
No. Man has not elevated himself, but the Lord has, by His infinite grace, made all believers "friends" and "brothers", as well as "kings" and "priests", "heirs of God, and JOINT-HEIRS with Christ". David will be "co-regent" [the word is not important, it is the idea] in the sense that he will reign over Israel under Christ.
No one can be equal with the King of Glory, the King of kings and the Lord of lords. But God, in His grace and kindness, gives His children the privilege to be "heirs of God" and "JOINT-HEIRS" with Christ. It is in that sense that David is "co-regent". So let's not start misinterpreting or misunderstanding God's grace.