Revelation 5:10 And hast made us unto our God kings and priests: and we shall reign on the earth.
The promise of our becoming kings and priests unto God is repeated in this verse, with the specific addition that we shall reign on the earth. When? How? In the future Judgment Day and Eternal Kingdom we will sit on thrones and judge. Exactly what that means, how that works, and who we will rule over remains somewhat of a mystery, but the promise is clear: in the new heaven and new earth Christians will be kings and rule. We will make authoritative decisions and carry them out. At least it means we will have positions of honor and responsibility. We will have been trained for our role by a lifetime of living in Christ.
This promise restores what mankind had in the past Paradise of Eden, but lost: “Subdue the earth and have dominion.” And Adam named the animals. Man was created to have dominion and rule over all of Creation in the place of God as a kind of vice-regent. But Satan legally usurped that authority and became the ”prince of this world.” Then the ”Second Adam” (Jesus) had to come and regain what the “First Adam” (we humans) had lost.
That leaves us believers as rulers with Christ in the present. By faith we rule with Christ over sin and death and all evil. Though it may appear to us that Sin, Death, and Evil are ruling the world and having their way, spiritually and inwardly in the hearts of Christians the ”kings and priests” are reigning. That’s the promise.