I find Eugene Merrill’s statement that the kingdom of God refers to “the sovereignty of God over all His creation, mediated through man, His vice-regent and image” important in its inclusion of “mediated through man.”
Scholars have long wrestled with the fact that God has never lost his sovereignty over the earth while the kingdom of God was something not present that drew near in Christ. For instance, Craig Blomberg explains this tension by proposing, “It was not that [God] was not previously king, but his sovereignty is now being demonstrated in a new, clearer, and more powerful fashion.” Thomas Schreiner presents a bit sharper explanation: “[Jesus] was not referring to God’s sovereign reign over all of history, for God has always ruled over all that occurs. The coming of the kingdom that Jesus proclaimed designated something new, a time when God’s enemies would be demonstrably defeated and the righteous would be visibly blessed.”
Blomberg and Schreiner are not wrong in what they say. But I don’t think their explanations go far enough. For instance, Jesus reigns now, but because he reigns “in the midst of his enemies” (Psalm 110:2) and does not yet “execute judgment among the nations” in “the day of his wrath” (Psalm 110:5-6), it does not seem that at present “God’s enemies [are] demonstrably defeated” or that “the righteous [are] visibly blessed.” We’re still in the time of “Blessed are those who are persecuted for righteousness’ sake, for theirs is the kingdom of heaven” (Matthew 5:10).
I think greater clarity comes when the fact that the kingdom theme has its roots in Genesis 1:26-28 is combined with the fact that the kingdom draws near with the Son’s incarnation. The kingdom is about God’s reign over the earth mediated through man. The marvelous thing about God’s plan is that the ultimate human ruler over the earth, Jesus, is both God and man in the same Person.
 Craig L. Blomberg, Jesus and the Gospels: An Introduction and Survey, 2nd ed. (Nashville: B&H, 2009), 271, n. 1.
 Thomas R. Schreiner, New Testament Theology: Magnifying God in Christ (Grand Rapids: Baker, 2008), 53-54.