And when he had taken the scroll, the four living creatures and the twenty-four elders fell down before the Lamb, each holding a harp, and golden bowls full of incense, which are the prayers of the saints.  And they sang a new song, saying,
"Worthy are you to take the scroll
and to open its seals,
for you were slain, and by your blood you ransomed people for God
from every tribe and language and people and nation,
 and you have made them a kingdom and priests to our God,
and they shall reign on the earth."
Rev. 5:8-10 (ESV)
Now we see the “living creatures” and the elders falling down and worshiping, with incense even, as the Lion in the form of a Lamb takes the scroll. He is worthy for he ransomed people for God of every nation and people and made them to be a royal priesthood, a kingdom and priests to our God who reigns so that we reign..
Yes Christians reign in this world. We reign in the same way Christ reigns. Not the way of earthly leaders and kings. But in the way of Jesus we reign in the world through the reign of Christ in our hearts, his kingdom marked by the forgiveness of sins, established by conquering death. If death has not hold on us, then neither does this world.