These two terms appear together in this verse as an echo of how these two terms appear side by side in Psalm 2:1–12. That psalm had said that “the kings of the earth set themselves…against the Lord and against his Anointed” (Psalm 2:2), and in the Greek translation of the original Hebrew the word “anointed” is literally the word “Christ.” Later in this same psalm the Lord said concerning his Son that he would make the nations his possession (Psalm 2:8) and challenged the rulers of the earth to kiss the Son (Psalm 2:12). The creatures of heaven allude to this psalm and declare its fulfillment in John’s hearing. The pronoun “his” in connection to Christ is not then intended to contrast “our Lord” from “his Christ” as if the reference is to two different persons. “His” picks up the formulation of Psalm 2:2.
15 Then the seventh angel blew his trumpet, and there were loud voices in heaven, saying, “The kingdom of the world has become the kingdom of our Lord and of his Christ, and he shall reign forever and ever.”