I am presently reading through Keith Mathison's "Postmillenialism: An Eschatology of Hope" and have been impressed thus far with both the thoroughness and depth of his scriptural argument as well as the fairness to the other eschatological camps. I of course come to the book already accepting a postmillenial position and just want to go further up and further in as C.S. Lewis put it. On the issue of whether Christ is reigning now or later, Postmillenialism shows itself to be the most Biblical here in regard to the throne matter.
"The LORD says to my Lord: "Sit at my right hand, until I make your enemies your footstool." (Psalm 110:1)
This is actually the most quoted Old Testament text in the New Testament. So, obviously the Spirit thought it to be important to our understanding of the New Covenant. In one of the New Testament references, Peter's sermon in Acts, we see the application of this passage:
"This Jesus God raised up, and of that we all are witnesses. Being therefore exalted at the right hand of God, and having received from the Father the promise of the Holy Spirit, he has poured out this that you yourselves are seeing and hearing.
For David did not ascend into the heavens, but he himself says, "'The Lord said to my Lord, Sit at my right hand, until I make your enemies your footstool.' Let all the house of Israel therefore know for certain that God has made him both Lord and Christ, this Jesus whom you crucified." (Acts 2:32-36)
We see here clearly the answer to the question in this post's title, when Christ ascended into heaven, He sat down at the right hand of God and there shall remain, until His enemies are put under His feet. Understanding this in a Postmillenial context has helped me to understand other scriptures. For example:
"Nevertheless, I tell you the truth: it is to your advantage that I go away, for if I do not go away, the Helper will not come to you. But if I go, I will send him to you." (John 16:7)
I never understood this passage when I had a Premillenial view of Christ's return, it seemed to me that it would be better if they rolled out the throne in Jerusalem and Christ began to reign in an earthly kingdom. If that indeed is the endgame as the Premils would say how in the world can it be better that Christ not set up an earthly throne now?
Ah, but that's the rub, He is on the throne, right now. At least that's what the Spirit said, and from there He rules the nations (Psalm 2) and shall see all things put under His feet.